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}\n      .ols-table tr { margin-bottom: 14px; border: 1px solid var(--border); border-radius: 18px; overflow: hidden; background: #ffffff; box-shadow: var(--inner-shadow); }\n      .ols-table td { border-bottom: 1px solid var(--border); padding: 14px 16px; overflow-wrap: anywhere; }\n      .ols-table td:last-child { border-bottom: none; }\n      .ols-table td::before { content: attr(data-label); display: block; margin-bottom: 6px; font-size: 13px; font-weight: 700; color: var(--blue); }\n      .ols-author { align-items: flex-start; padding: 22px 18px; border-radius: 22px; }\n      .ols-author-avatar-img { width: 72px; height: 72px; }\n      .ols-author-title { font-size: 24px; }\n      .ols-author-description { font-size: 15px; }\n    }\n  <\/style>\n\n  <div class=\"ols-revision-layout\">\n    <aside class=\"ols-sidebar\">\r\n  <div class=\"ols-sidebar-header\">\r\n    <h3>Revision Notes<\/h3>\r\n    <p>A Level Chemistry<\/p>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Topic 2 Atomic Structure and The Periodic Table<\/h4>\r\n\r\n    <ul class=\"ols-topic-list\">\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/atomic-structure\/\">Atomic Structure<\/a>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/mass-spectrometry\/\">Mass Spectrometry<\/a>\r\n\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/mass-spectrometry\/the-mass-spectrometer\/\">The Mass Spectrometer<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/mass-spectrometry\/ram-calculations\/\">RAM Calculations<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/mass-spectrometry\/predicting-diatomic-mass-spectra\/\">Predicting Diatomic Mass Spectra<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/mass-spectrometry\/identifying-compounds-from-mass-spectra\/\">Identifying Compounds from Mass Spectra<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/quantum-model\/\">Quantum Model<\/a>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/\">Ionisation Energy<\/a>\r\n\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/introduction-to-ionisation-energy\/\">Introduction to Ionisation Energy<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/successive-ionisation-energy\/\">Successive Ionisation Energy<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/trends-in-a-period\/\">Trends in a Period<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/trends-in-a-group\/\">Trends in a Group<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Next Topics<\/h4>\r\n\r\n    <ul class=\"ols-topic-list\">\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-3-bonding-structure\/\">Bonding &amp; Structure<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-4-organic-chemistry-and-alkanes\/\">Organic Chemistry &amp; Alkanes<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/\">Alkenes<\/a>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/div>\r\n<\/aside>\r\n\r\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n    return String(path || '')\r\n      .split('?')[0]\r\n      .split('#')[0]\r\n      .replace(\/\\\/+$\/, '')\r\n      .toLowerCase();\r\n  }\r\n\r\n  function highlightActive() {\r\n    var sidebar = document.querySelector('.ols-sidebar');\r\n    if (!sidebar) return false;\r\n\r\n    var currentPath = normalisePath(window.location.pathname);\r\n    var links = sidebar.querySelectorAll('a[href]');\r\n    var matched = null;\r\n    var matchedLength = 0;\r\n\r\n    sidebar.querySelectorAll('.active, .active-main, .parent-active').forEach(function(item) {\r\n      item.classList.remove('active', 'active-main', 'parent-active');\r\n    });\r\n\r\n    sidebar.querySelectorAll('a[data-ols-disabled-parent=\"1\"], a[aria-current]').forEach(function(a) {\r\n      a.removeAttribute('aria-current');\r\n      a.removeAttribute('tabindex');\r\n      a.removeAttribute('data-ols-disabled-parent');\r\n    });\r\n\r\n    links.forEach(function(a) {\r\n      try {\r\n        var href = a.getAttribute('href');\r\n\r\n        if (!href || href === '#' || href.charAt(0) === '#') {\r\n          return;\r\n        }\r\n\r\n        var linkPath = normalisePath(new URL(href, window.location.origin).pathname);\r\n\r\n        if (!linkPath) {\r\n          return;\r\n        }\r\n\r\n        if (currentPath === linkPath || currentPath.indexOf(linkPath + '\/') === 0) {\r\n          if (linkPath.length > matchedLength) {\r\n            matched = a;\r\n            matchedLength = linkPath.length;\r\n          }\r\n        }\r\n      } catch(e) {}\r\n    });\r\n\r\n    if (matched) {\r\n      var matchedLi = matched.closest('li');\r\n      var parentSub = matched.closest('.ols-subtopic-list');\r\n\r\n      if (parentSub) {\r\n        if (matchedLi) {\r\n          matchedLi.classList.add('active');\r\n          matched.setAttribute('aria-current', 'page');\r\n        }\r\n\r\n        var parentLi = parentSub.closest('li');\r\n\r\n        if (parentLi) {\r\n          parentLi.classList.add('parent-active', 'active-main');\r\n\r\n          var parentLink = parentLi.querySelector(':scope > a');\r\n\r\n          if (parentLink) {\r\n            parentLink.setAttribute('aria-current', 'true');\r\n            parentLink.setAttribute('tabindex', '-1');\r\n            parentLink.setAttribute('data-ols-disabled-parent', '1');\r\n          }\r\n        }\r\n      } else {\r\n        if (matchedLi) {\r\n          matchedLi.classList.add('parent-active', 'active-main');\r\n          matched.setAttribute('aria-current', 'page');\r\n          matched.setAttribute('tabindex', '-1');\r\n          matched.setAttribute('data-ols-disabled-parent', '1');\r\n        }\r\n      }\r\n    }\r\n\r\n    return true;\r\n  }\r\n\r\n  if (!highlightActive()) {\r\n    document.addEventListener('DOMContentLoaded', highlightActive);\r\n  }\r\n})();\r\n<\/script>\r\n\n\n\n    <main class=\"ols-main\">\n      <nav class=\"ols-breadcrumbs\" aria-label=\"Breadcrumb\">\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/\">Revision Notes<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/\">A Level Chemistry<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Edexcel International<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/\">Topic 2 Atomic Structure &amp; The Periodic Table<\/a> \/\n        <span>Quantum Model<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Quantum Model<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to shells, subshells, orbitals, electronic configuration, Hund&#8217;s rule and the Pauli exclusion principle for Edexcel International A Level Chemistry.<\/p>\n        <div class=\"ols-badges\">\n          <div class=\"ols-badge\">Exam board: Edexcel International<\/div>\n          <div class=\"ols-badge\">Topic 2: Atomic Structure &amp; The Periodic Table<\/div>\n          <div class=\"ols-badge\">Level: A Level Chemistry<\/div>\n        <\/div>\n        <div class=\"ols-author\">\n          <img decoding=\"async\" class=\"ols-author-avatar-img\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\" alt=\"Dr. Mohammed Al-Fatah\">\n          <div class=\"ols-author-content\">\n            <h2 class=\"ols-author-title\">Written by:<br><span>Dr. Mohammed Al-Fatah<\/span><\/h2>\n            <p class=\"ols-author-description\">Chemistry specialist revision notes for A Level Chemistry.<\/p>\n            <a class=\"ols-linkedin-pill\" href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\" target=\"_blank\" rel=\"noopener noreferrer\">\n              <svg class=\"ols-linkedin-icon\" viewBox=\"0 0 24 24\" fill=\"currentColor\" aria-hidden=\"true\"><path d=\"M4.98 3.5C4.98 4.88 3.86 6 2.48 6S0 4.88 0 3.5 1.12 1 2.48 1s2.5 1.12 2.5 2.5zM.5 8h4V24h-4V8zm7 0h3.8v2.2h.1c.5-.9 1.8-2.2 3.9-2.2 4.2 0 5 2.8 5 6.4V24h-4v-7.6c0-1.8 0-4.2-2.6-4.2s-3 2-3 4v7.8h-4V8z\"><\/path><\/svg>\n              View LinkedIn Profile\n            <\/a>\n          <\/div>\n        <\/div>\n      <\/header>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">1<\/div><h2>From the Bohr Model to the Quantum Model<\/h2><\/div>\n        <p>At GCSE level, electrons are often shown moving in circular or spherical <strong>shells<\/strong> around the nucleus. This early model is useful because it explains why noble gas arrangements are particularly stable.<\/p>\n        <p>At A Level, the model becomes more detailed. Electrons are not treated as tiny particles moving in fixed paths. Instead, an <strong>orbital<\/strong> describes a region of space where there is a high probability of finding an electron.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Key idea:<\/strong> The quantum model replaces fixed electron orbits with probability-based orbitals.<\/p><\/div>\n      <\/article>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">2<\/div><h2>Shells, Subshells and Orbitals<\/h2><\/div>\n        <p>The quantum model describes electron arrangement using three linked levels of organisation: <strong>shells<\/strong>, <strong>subshells<\/strong> and <strong>orbitals<\/strong>.<\/p>\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr><th>Term<\/th><th>Meaning<\/th><th>Key notation<\/th><\/tr>\n            <\/thead>\n            <tbody>\n              <tr><td data-label=\"Term\"><strong>Shell<\/strong><\/td><td data-label=\"Meaning\">A principal energy level around the nucleus.<\/td><td data-label=\"Key notation\">Defined by the principal quantum number, <strong>n<\/strong>.<\/td><\/tr>\n              <tr><td data-label=\"Term\"><strong>Subshell<\/strong><\/td><td data-label=\"Meaning\">A subdivision of a shell. Common subshells are s, p, d and f.<\/td><td data-label=\"Key notation\">Defined by the angular momentum quantum number, <strong>l<\/strong>.<\/td><\/tr>\n              <tr><td data-label=\"Term\"><strong>Orbital<\/strong><\/td><td data-label=\"Meaning\">A specific region in a subshell where an electron is most likely to be found.<\/td><td data-label=\"Key notation\">Each orbital can hold a maximum of <strong>2 electrons<\/strong>.<\/td><\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n        <div class=\"ols-key-box\"><p><strong>Exam focus:<\/strong> Avoid saying that an orbital is a shell. A shell contains subshells, and subshells contain orbitals.<\/p><\/div>\n      <\/article>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">3<\/div><h2>s, p and d Orbital Shapes<\/h2><\/div>\n        <p>Orbitals describe the <strong>mathematical probability<\/strong> of locating an electron in a region around the nucleus. Each orbital has a three-dimensional shape, but these shapes are approximations that help us visualise electron distribution.<\/p>\n        <p>The <strong>s orbital<\/strong> is spherical. The <strong>p orbitals<\/strong> are dumbbell-shaped and are arranged at right angles to each other. The <strong>d orbitals<\/strong> have more complex shapes, with four of the five d orbitals having a four-lobed cloverleaf arrangement.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Remember:<\/strong> Orbital diagrams are models. They show regions of high electron probability, not solid surfaces or fixed paths.<\/p><\/div>\n      <\/article>\n\n      <section class=\"ols-s-orbital-card\" id=\"ols-s-orbital-card\">\n  <style>\n    #ols-s-orbital-card {\n      width: 100%;\n      margin: 28px auto;\n      font-family: Poppins, Arial, sans-serif;\n      color: #1C244B;\n    }\n\n    #ols-s-orbital-card * {\n      box-sizing: border-box;\n    }\n\n    #ols-s-orbital-card .ols-s-card {\n      width: 100%;\n      background: #ffffff;\n      border: 1px solid rgba(28, 36, 75, 0.10);\n      border-radius: 26px;\n      box-shadow: 0 14px 36px rgba(28, 36, 75, 0.10);\n      overflow: hidden;\n    }\n\n    #ols-s-orbital-card .ols-s-header {\n      padding: 28px 30px 18px;\n      background: linear-gradient(135deg, #f8fbff 0%, #eef4ff 100%);\n      border-bottom: 1px solid rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-s-orbital-card .ols-s-pill {\n      display: inline-flex;\n      align-items: center;\n      gap: 8px;\n     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box-shadow 0.2s ease, background 0.2s ease;\n    }\n\n    #ols-s-orbital-card .ols-s-btn:hover {\n      transform: translateY(-1px);\n      box-shadow: 0 12px 24px rgba(28, 36, 75, 0.22);\n      background: #27315e;\n    }\n\n    #ols-s-orbital-card .ols-s-btn.secondary {\n      color: #1C244B;\n      background: #EEF4FF;\n      border: 1px solid rgba(28, 36, 75, 0.12);\n      box-shadow: none;\n    }\n\n    #ols-s-orbital-card .ols-s-btn.secondary:hover {\n      background: #e4edff;\n      box-shadow: none;\n      transform: translateY(-1px);\n    }\n\n    #ols-s-orbital-card .ols-s-info-grid {\n      display: grid;\n      grid-template-columns: repeat(4, minmax(0, 1fr));\n      gap: 16px;\n      padding: 22px;\n      background: #f8fbff;\n      border-top: 1px solid rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-s-orbital-card .ols-s-info-box {\n      background: #ffffff;\n      border: 1px solid rgba(28, 36, 75, 0.10);\n      border-radius: 20px;\n      padding: 18px 18px 16px;\n      box-shadow: 0 8px 22px rgba(28, 36, 75, 0.06);\n    }\n\n    #ols-s-orbital-card .ols-s-info-box h3 {\n      margin: 0 0 8px;\n      font-size: 17px;\n      line-height: 1.25;\n      font-weight: 700;\n      color: #1C244B;\n    }\n\n    #ols-s-orbital-card .ols-s-info-box p {\n      margin: 0;\n      font-size: 15px;\n      line-height: 1.55;\n      font-weight: 300;\n      color: rgba(28, 36, 75, 0.82);\n    }\n\n    #ols-s-orbital-card .ols-key-blue {\n      color: #2166c2;\n      font-weight: 700;\n    }\n\n    @media (max-width: 1000px) {\n      #ols-s-orbital-card .ols-s-info-grid {\n        grid-template-columns: repeat(2, minmax(0, 1fr));\n      }\n    }\n\n    @media (max-width: 900px) {\n      #ols-s-orbital-card .ols-s-viewer-wrap {\n        height: 500px;\n      }\n    }\n\n    @media (max-width: 600px) {\n      #ols-s-orbital-card .ols-s-header {\n        padding: 24px 20px 16px;\n      }\n\n      #ols-s-orbital-card .ols-s-subtitle {\n        font-size: 16px;\n      }\n\n      #ols-s-orbital-card .ols-s-viewer-wrap {\n        height: 430px;\n      }\n\n      #ols-s-orbital-card .ols-s-controls {\n        justify-content: stretch;\n      }\n\n      #ols-s-orbital-card .ols-s-btn {\n        width: 100%;\n      }\n\n      #ols-s-orbital-card .ols-s-info-grid {\n        grid-template-columns: 1fr;\n      }\n    }\n  <\/style>\n\n  <div class=\"ols-s-card\">\n    <div class=\"ols-s-header\">\n      <div class=\"ols-s-pill\">Atomic orbitals \u2022 s orbital<\/div>\n      <h2 class=\"ols-s-title\">The s Orbital<\/h2>\n      <p class=\"ols-s-subtitle\">\n        This 3D model shows an <strong>s orbital<\/strong>. Unlike a p orbital, an s orbital is <strong>spherical<\/strong>, meaning the electron density is distributed equally in all directions around the nucleus. It has no directional lobes and no nodal plane.\n      <\/p>\n    <\/div>\n\n    <div class=\"ols-s-viewer-wrap\">\n      <div id=\"olsSOrbitalViewer\"><\/div>\n    <\/div>\n\n    <div class=\"ols-s-controls\">\n      <button class=\"ols-s-btn\" id=\"olsSOrbitalToggleSpin\" type=\"button\">Pause rotation<\/button>\n      <button class=\"ols-s-btn secondary\" id=\"olsSOrbitalResetView\" type=\"button\">Reset view<\/button>\n    <\/div>\n\n    <div class=\"ols-s-info-grid\">\n      <div class=\"ols-s-info-box\">\n        <h3>Shape<\/h3>\n        <p>\n          The <span class=\"ols-key-blue\">s orbital<\/span> is spherical, so it looks the same from every direction.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-s-info-box\">\n        <h3>Symmetry<\/h3>\n        <p>\n          Electron density is spread uniformly around the centre, giving the orbital spherical symmetry.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-s-info-box\">\n        <h3>No nodal plane<\/h3>\n        <p>\n          Unlike p orbitals, an s orbital does not have two separate lobes or a nodal plane passing through the centre.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-s-info-box\">\n        <h3>Key idea<\/h3>\n        <p>\n          The s orbital is the simplest atomic orbital and is non-directional because it has the same shape in all spatial directions.\n        <\/p>\n      <\/div>\n    <\/div>\n  <\/div>\n\n  <script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/S-Orbital.js\"><\/script>\n  <p style=\"text-align:center;font-size:11px;color:#aab0c0;font-style:italic;margin:0;padding:10px 0;\">\u00a9 Dr. Mohammed Al-Fatah \u00b7 <a href=\"https:\/\/onlinelearningsystem.net\" style=\"color:#aab0c0;text-decoration:none;\">onlinelearningsystem.net<\/a><\/p>\n<\/section>\n\n      <section class=\"ols-px-orbital-card\" id=\"ols-px-orbital-card\">\n  <style>\n    #ols-px-orbital-card {\n      width: 100%;\n      margin: 28px auto;\n      font-family: Poppins, Arial, sans-serif;\n      color: #1C244B;\n    }\n\n    #ols-px-orbital-card * {\n      box-sizing: border-box;\n    }\n\n    #ols-px-orbital-card .ols-px-card {\n      width: 100%;\n      background: #ffffff;\n      border: 1px solid rgba(28, 36, 75, 0.10);\n      border-radius: 26px;\n      box-shadow: 0 14px 36px rgba(28, 36, 75, 0.10);\n      overflow: hidden;\n    }\n\n    #ols-px-orbital-card .ols-px-header {\n      padding: 28px 30px 18px;\n      background: linear-gradient(135deg, #f8fbff 0%, #eef4ff 100%);\n      border-bottom: 1px solid rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-px-orbital-card .ols-px-pill {\n      display: inline-flex;\n      align-items: center;\n      gap: 8px;\n      background: rgba(200, 30, 30, 0.10);\n      color: #c81e1e;\n      border: 1px solid rgba(200, 30, 30, 0.16);\n      border-radius: 999px;\n      padding: 8px 14px;\n      font-size: 13px;\n      font-weight: 700;\n      letter-spacing: 0.02em;\n      margin-bottom: 14px;\n    }\n\n    #ols-px-orbital-card .ols-px-title {\n      margin: 0;\n      font-size: clamp(28px, 4vw, 40px);\n      line-height: 1.12;\n      font-weight: 600;\n      color: #1C244B;\n      text-shadow: -9px 0 9px rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-px-orbital-card .ols-px-subtitle {\n      margin: 12px 0 0;\n      max-width: 900px;\n      font-size: 18px;\n      line-height: 1.55;\n      font-weight: 300;\n      color: rgba(28, 36, 75, 0.82);\n    }\n\n    #ols-px-orbital-card .ols-px-viewer-wrap {\n      position: relative;\n      width: 100%;\n      height: 560px;\n      background:\n        radial-gradient(circle at 50% 45%, rgba(238, 244, 255, 0.95), rgba(255, 255, 255, 1) 58%),\n        linear-gradient(180deg, #ffffff 0%, #f8fbff 100%);\n      overflow: hidden;\n    }\n\n    #ols-px-orbital-card #olsPxOrbitalViewer {\n      width: 100%;\n      height: 100%;\n      display: block;\n      cursor: grab;\n    }\n\n    #ols-px-orbital-card #olsPxOrbitalViewer:active {\n      cursor: grabbing;\n    }\n\n    #ols-px-orbital-card .ols-px-controls {\n      display: flex;\n      flex-wrap: wrap;\n      gap: 12px;\n      align-items: center;\n      justify-content: center;\n      padding: 18px 20px;\n      background: #ffffff;\n      border-top: 1px solid rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-px-orbital-card .ols-px-btn {\n      appearance: none;\n      border: 0;\n      border-radius: 999px;\n      padding: 11px 18px;\n      font-family: Poppins, Arial, sans-serif;\n      font-size: 14px;\n      font-weight: 700;\n      color: #ffffff;\n      background: #1C244B;\n      box-shadow: 0 8px 18px rgba(28, 36, 75, 0.18);\n      cursor: pointer;\n      transition: transform 0.2s ease, box-shadow 0.2s ease, background 0.2s ease;\n    }\n\n    #ols-px-orbital-card .ols-px-btn:hover {\n      transform: translateY(-1px);\n      box-shadow: 0 12px 24px rgba(28, 36, 75, 0.22);\n      background: #27315e;\n    }\n\n    #ols-px-orbital-card .ols-px-btn.secondary {\n      color: #1C244B;\n      background: #EEF4FF;\n      border: 1px solid rgba(28, 36, 75, 0.12);\n      box-shadow: none;\n    }\n\n    #ols-px-orbital-card .ols-px-btn.secondary:hover {\n      background: #e4edff;\n      box-shadow: none;\n      transform: translateY(-1px);\n    }\n\n    #ols-px-orbital-card .ols-px-info-grid {\n      display: grid;\n      grid-template-columns: repeat(3, minmax(0, 1fr));\n      gap: 16px;\n      padding: 22px;\n      background: #f8fbff;\n      border-top: 1px solid rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-px-orbital-card .ols-px-info-box {\n      background: #ffffff;\n      border: 1px solid rgba(28, 36, 75, 0.10);\n      border-radius: 20px;\n      padding: 18px 18px 16px;\n      box-shadow: 0 8px 22px rgba(28, 36, 75, 0.06);\n    }\n\n    #ols-px-orbital-card .ols-px-info-box h3 {\n      margin: 0 0 8px;\n      font-size: 17px;\n      line-height: 1.25;\n      font-weight: 700;\n      color: #1C244B;\n    }\n\n    #ols-px-orbital-card .ols-px-info-box p {\n      margin: 0;\n      font-size: 15px;\n      line-height: 1.55;\n      font-weight: 300;\n      color: rgba(28, 36, 75, 0.82);\n    }\n\n    #ols-px-orbital-card .ols-px-key-red {\n      color: #c81e1e;\n      font-weight: 700;\n    }\n\n    #ols-px-orbital-card .ols-px-key-blue {\n      color: #2166c2;\n      font-weight: 700;\n    }\n\n    @media (max-width: 900px) {\n      #ols-px-orbital-card .ols-px-info-grid {\n        grid-template-columns: 1fr;\n      }\n\n      #ols-px-orbital-card .ols-px-viewer-wrap {\n        height: 500px;\n      }\n    }\n\n    @media (max-width: 600px) {\n      #ols-px-orbital-card .ols-px-header {\n        padding: 24px 20px 16px;\n      }\n\n      #ols-px-orbital-card .ols-px-subtitle {\n        font-size: 16px;\n      }\n\n      #ols-px-orbital-card .ols-px-viewer-wrap {\n        height: 430px;\n      }\n\n      #ols-px-orbital-card .ols-px-controls {\n        justify-content: stretch;\n      }\n\n      #ols-px-orbital-card .ols-px-btn {\n        width: 100%;\n      }\n    }\n  <\/style>\n\n  <div class=\"ols-px-card\">\n    <div class=\"ols-px-header\">\n      <div class=\"ols-px-pill\">Atomic orbitals \u2022 p<sub>x<\/sub> orbital<\/div>\n      <h2 class=\"ols-px-title\">p<sub>x<\/sub> Orbital Electron Cloud<\/h2>\n      <p class=\"ols-px-subtitle\">\n        A p<sub>x<\/sub> orbital has two lobes of electron density aligned along the x-axis. The blue transparent plane shows the nodal plane where the probability of finding the electron is zero.\n      <\/p>\n    <\/div>\n\n    <div class=\"ols-px-viewer-wrap\">\n      <div id=\"olsPxOrbitalViewer\"><\/div>\n    <\/div>\n\n    <div class=\"ols-px-controls\">\n      <button class=\"ols-px-btn\" id=\"olsPxToggleSpin\" type=\"button\">Pause rotation<\/button>\n      <button class=\"ols-px-btn secondary\" id=\"olsPxResetView\" type=\"button\">Reset view<\/button>\n    <\/div>\n\n    <div class=\"ols-px-info-grid\">\n      <div class=\"ols-px-info-box\">\n        <h3>Orientation<\/h3>\n        <p>\n          The <strong>p<sub>x<\/sub><\/strong> orbital is aligned along the <strong>x-axis<\/strong>, so the two lobes point left and right.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-px-info-box\">\n        <h3>Electron cloud density<\/h3>\n        <p>\n          The <span class=\"ols-px-key-red\">red translucent lobes<\/span> represent regions where electron density is concentrated.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-px-info-box\">\n        <h3>Nodal plane<\/h3>\n        <p>\n          The <span class=\"ols-px-key-blue\">blue transparent YZ plane<\/span> passes through the origin and separates the two lobes.\n        <\/p>\n      <\/div>\n    <\/div>\n  <\/div>\n\n  <script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/Px_Orbital.js\"><\/script>\n  <p style=\"text-align:center;font-size:11px;color:#aab0c0;font-style:italic;margin:0;padding:10px 0;\">\u00a9 Dr. Mohammed Al-Fatah \u00b7 <a href=\"https:\/\/onlinelearningsystem.net\" style=\"color:#aab0c0;text-decoration:none;\">onlinelearningsystem.net<\/a><\/p>\n<\/section>\n\n      <section class=\"ols-p-orbitals-card\" id=\"ols-p-orbitals-card\">\n  <style>\n    #ols-p-orbitals-card {\n      width: 100%;\n      margin: 28px auto;\n      font-family: Poppins, Arial, sans-serif;\n      color: #1C244B;\n    }\n\n    #ols-p-orbitals-card * {\n      box-sizing: border-box;\n    }\n\n    #ols-p-orbitals-card .ols-p-card {\n      width: 100%;\n      background: #ffffff;\n      border: 1px solid rgba(28, 36, 75, 0.10);\n      border-radius: 26px;\n      box-shadow: 0 14px 36px rgba(28, 36, 75, 0.10);\n      overflow: hidden;\n    }\n\n    #ols-p-orbitals-card .ols-p-header {\n      padding: 28px 30px 18px;\n      background: linear-gradient(135deg, #f8fbff 0%, #eef4ff 100%);\n      border-bottom: 1px solid rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-p-orbitals-card .ols-p-pill {\n      display: inline-flex;\n      align-items: center;\n      gap: 8px;\n      background: rgba(200, 30, 30, 0.10);\n      color: #c81e1e;\n      border: 1px solid rgba(200, 30, 30, 0.16);\n      border-radius: 999px;\n      padding: 8px 14px;\n      font-size: 13px;\n      font-weight: 700;\n      letter-spacing: 0.02em;\n      margin-bottom: 14px;\n    }\n\n    #ols-p-orbitals-card .ols-p-title {\n      margin: 0;\n      font-size: clamp(28px, 4vw, 40px);\n      line-height: 1.12;\n      font-weight: 600;\n      color: #1C244B;\n      text-shadow: -9px 0 9px rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-p-orbitals-card .ols-p-subtitle {\n      margin: 12px 0 0;\n      max-width: 980px;\n      font-size: 18px;\n      line-height: 1.55;\n      font-weight: 300;\n      color: rgba(28, 36, 75, 0.82);\n    }\n\n    #ols-p-orbitals-card .ols-p-viewer-wrap {\n      position: relative;\n      width: 100%;\n      height: 560px;\n      background:\n        radial-gradient(circle at 50% 45%, rgba(238, 244, 255, 0.95), rgba(255, 255, 255, 1) 58%),\n        linear-gradient(180deg, #ffffff 0%, #f8fbff 100%);\n      overflow: hidden;\n    }\n\n    #ols-p-orbitals-card #olsPOrbitalsViewer {\n      width: 100%;\n      height: 100%;\n      display: block;\n      cursor: grab;\n    }\n\n    #ols-p-orbitals-card #olsPOrbitalsViewer:active {\n      cursor: grabbing;\n    }\n\n    #ols-p-orbitals-card .ols-p-controls {\n      display: flex;\n      flex-wrap: wrap;\n      gap: 12px;\n      align-items: center;\n      justify-content: center;\n      padding: 18px 20px;\n      background: #ffffff;\n      border-top: 1px solid rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-p-orbitals-card .ols-p-btn {\n      appearance: none;\n      border: 0;\n      border-radius: 999px;\n      padding: 11px 18px;\n      font-family: Poppins, Arial, sans-serif;\n      font-size: 14px;\n      font-weight: 700;\n      color: #ffffff;\n      background: #1C244B;\n      box-shadow: 0 8px 18px rgba(28, 36, 75, 0.18);\n      cursor: pointer;\n      transition: transform 0.2s ease, box-shadow 0.2s ease, background 0.2s ease, color 0.2s ease, border-color 0.2s ease;\n      border: 1px solid transparent;\n    }\n\n    #ols-p-orbitals-card .ols-p-btn:hover {\n      transform: translateY(-1px);\n      box-shadow: 0 12px 24px rgba(28, 36, 75, 0.22);\n      background: #27315e;\n    }\n\n    #ols-p-orbitals-card .ols-p-btn.secondary {\n      color: #1C244B;\n      background: #EEF4FF;\n      border: 1px solid rgba(28, 36, 75, 0.12);\n      box-shadow: none;\n    }\n\n    #ols-p-orbitals-card .ols-p-btn.secondary:hover {\n      background: #e4edff;\n      box-shadow: none;\n      transform: translateY(-1px);\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn {\n      box-shadow: none;\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn:hover {\n      box-shadow: none;\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn.px {\n      background: #c81e1e;\n      color: #ffffff;\n      border-color: #c81e1e;\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn.px:hover {\n      background: #b11a1a;\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn.py {\n      background: #7a3db8;\n      color: #ffffff;\n      border-color: #7a3db8;\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn.py:hover {\n      background: #6b35a1;\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn.pz {\n      background: #2f8f46;\n      color: #ffffff;\n      border-color: #2f8f46;\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn.pz:hover {\n      background: #297c3d;\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn.inactive {\n      background: #ffffff;\n      box-shadow: none;\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn.inactive:hover {\n      background: #f8fbff;\n      box-shadow: none;\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn.px.inactive {\n      color: #c81e1e;\n      border-color: rgba(200, 30, 30, 0.30);\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn.py.inactive {\n      color: #7a3db8;\n      border-color: rgba(122, 61, 184, 0.30);\n    }\n\n    #ols-p-orbitals-card .ols-p-orbital-btn.pz.inactive {\n      color: #2f8f46;\n      border-color: rgba(47, 143, 70, 0.30);\n    }\n\n    #ols-p-orbitals-card .ols-p-info-grid {\n      display: grid;\n      grid-template-columns: repeat(4, minmax(0, 1fr));\n      gap: 16px;\n      padding: 22px;\n      background: #f8fbff;\n      border-top: 1px solid rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-p-orbitals-card .ols-p-info-box {\n      background: #ffffff;\n      border: 1px solid rgba(28, 36, 75, 0.10);\n      border-radius: 20px;\n      padding: 18px 18px 16px;\n      box-shadow: 0 8px 22px rgba(28, 36, 75, 0.06);\n    }\n\n    #ols-p-orbitals-card .ols-p-info-box h3 {\n      margin: 0 0 8px;\n      font-size: 17px;\n      line-height: 1.25;\n      font-weight: 700;\n      color: #1C244B;\n    }\n\n    #ols-p-orbitals-card .ols-p-info-box p {\n      margin: 0;\n      font-size: 15px;\n      line-height: 1.55;\n      font-weight: 300;\n      color: rgba(28, 36, 75, 0.82);\n    }\n\n    #ols-p-orbitals-card .ols-key-red {\n      color: #c81e1e;\n      font-weight: 700;\n    }\n\n    #ols-p-orbitals-card .ols-key-purple {\n      color: #7a3db8;\n      font-weight: 700;\n    }\n\n    #ols-p-orbitals-card .ols-key-green {\n      color: #2f8f46;\n      font-weight: 700;\n    }\n\n    @media (max-width: 1000px) {\n      #ols-p-orbitals-card .ols-p-info-grid {\n        grid-template-columns: repeat(2, minmax(0, 1fr));\n      }\n    }\n\n    @media (max-width: 900px) {\n      #ols-p-orbitals-card .ols-p-viewer-wrap {\n        height: 500px;\n      }\n    }\n\n    @media (max-width: 600px) {\n      #ols-p-orbitals-card .ols-p-header {\n        padding: 24px 20px 16px;\n      }\n\n      #ols-p-orbitals-card .ols-p-subtitle {\n        font-size: 16px;\n      }\n\n      #ols-p-orbitals-card .ols-p-viewer-wrap {\n        height: 430px;\n      }\n\n      #ols-p-orbitals-card .ols-p-controls {\n        justify-content: stretch;\n      }\n\n      #ols-p-orbitals-card .ols-p-btn {\n        width: 100%;\n      }\n\n      #ols-p-orbitals-card .ols-p-info-grid {\n        grid-template-columns: 1fr;\n      }\n    }\n  <\/style>\n\n  <div class=\"ols-p-card\">\n    <div class=\"ols-p-header\">\n      <div class=\"ols-p-pill\">Atomic orbitals \u2022 p orbitals<\/div>\n      <h2 class=\"ols-p-title\">The Three p Orbitals<\/h2>\n      <p class=\"ols-p-subtitle\">\n        This 3D model shows the three p orbitals together. The <span class=\"ols-key-red\">p<sub>x<\/sub><\/span> orbital is shown in red, the <span class=\"ols-key-purple\">p<sub>y<\/sub><\/span> orbital in purple, and the <span class=\"ols-key-green\">p<sub>z<\/sub><\/span> orbital in green. Each orbital consists of two lobes that meet at the origin, and the three orbitals are arranged at right angles to one another.\n      <\/p>\n    <\/div>\n\n    <div class=\"ols-p-viewer-wrap\">\n      <div id=\"olsPOrbitalsViewer\"><\/div>\n    <\/div>\n\n    <div class=\"ols-p-controls\">\n      <button class=\"ols-p-btn\" id=\"olsPOrbitalsToggleSpin\" type=\"button\">Pause rotation<\/button>\n      <button class=\"ols-p-btn secondary\" id=\"olsPOrbitalsResetView\" type=\"button\">Reset view<\/button>\n      <button class=\"ols-p-btn ols-p-orbital-btn px\" id=\"olsTogglePx\" type=\"button\" aria-pressed=\"true\">Hide p<sub>x<\/sub><\/button>\n      <button class=\"ols-p-btn ols-p-orbital-btn py\" id=\"olsTogglePy\" type=\"button\" aria-pressed=\"true\">Hide p<sub>y<\/sub><\/button>\n      <button class=\"ols-p-btn ols-p-orbital-btn pz\" id=\"olsTogglePz\" type=\"button\" aria-pressed=\"true\">Hide p<sub>z<\/sub><\/button>\n    <\/div>\n\n    <div class=\"ols-p-info-grid\">\n      <div class=\"ols-p-info-box\">\n        <h3>p<sub>x<\/sub> orbital<\/h3>\n        <p>\n          The <span class=\"ols-key-red\">red<\/span> orbital lies along the x-axis.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-p-info-box\">\n        <h3>p<sub>y<\/sub> orbital<\/h3>\n        <p>\n          The <span class=\"ols-key-purple\">purple<\/span> orbital lies along the y-axis.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-p-info-box\">\n        <h3>p<sub>z<\/sub> orbital<\/h3>\n        <p>\n          The <span class=\"ols-key-green\">green<\/span> orbital lies along the z-axis.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-p-info-box\">\n        <h3>Key idea<\/h3>\n        <p>\n          All three p orbitals have the same shape and size. They differ only in their orientation in space.\n        <\/p>\n      <\/div>\n    <\/div>\n  <\/div>\n\n  <script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/PxPyPz_Orbitals.js\"><\/script>\n  <p style=\"text-align:center;font-size:11px;color:#aab0c0;font-style:italic;margin:0;padding:10px 0;\">\u00a9 Dr. Mohammed Al-Fatah \u00b7 <a href=\"https:\/\/onlinelearningsystem.net\" style=\"color:#aab0c0;text-decoration:none;\">onlinelearningsystem.net<\/a><\/p>\n<\/section>\n\n      <section class=\"ols-d-orbitals-card\" id=\"ols-d-orbitals-card\">\n  <style>\n    #ols-d-orbitals-card {\n      width: 100%;\n      margin: 28px auto;\n      font-family: Poppins, Arial, sans-serif;\n      color: #1C244B;\n    }\n\n    #ols-d-orbitals-card * {\n      box-sizing: border-box;\n    }\n\n    #ols-d-orbitals-card .ols-d-card {\n      width: 100%;\n      background: #ffffff;\n      border: 1px solid rgba(28, 36, 75, 0.10);\n      border-radius: 26px;\n      box-shadow: 0 14px 36px rgba(28, 36, 75, 0.10);\n      overflow: hidden;\n    }\n\n    #ols-d-orbitals-card .ols-d-header {\n      padding: 28px 30px 18px;\n      background: linear-gradient(135deg, #f8fbff 0%, #eef4ff 100%);\n      border-bottom: 1px solid rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-d-orbitals-card .ols-d-pill {\n      display: inline-flex;\n      align-items: center;\n      gap: 8px;\n      background: rgba(122, 61, 184, 0.10);\n      color: #7a3db8;\n      border: 1px solid rgba(122, 61, 184, 0.16);\n      border-radius: 999px;\n      padding: 8px 14px;\n      font-size: 13px;\n      font-weight: 700;\n      letter-spacing: 0.02em;\n      margin-bottom: 14px;\n    }\n\n    #ols-d-orbitals-card .ols-d-title {\n      margin: 0;\n      font-size: clamp(28px, 4vw, 40px);\n      line-height: 1.12;\n      font-weight: 600;\n      color: #1C244B;\n      text-shadow: -9px 0 9px rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-d-orbitals-card .ols-d-subtitle {\n      margin: 12px 0 0;\n      max-width: 980px;\n      font-size: 18px;\n      line-height: 1.55;\n      font-weight: 300;\n      color: rgba(28, 36, 75, 0.82);\n    }\n\n    #ols-d-orbitals-card .ols-d-viewer-wrap {\n      position: relative;\n      width: 100%;\n      height: 560px;\n      background:\n        radial-gradient(circle at 50% 45%, rgba(238, 244, 255, 0.95), rgba(255, 255, 255, 1) 58%),\n        linear-gradient(180deg, #ffffff 0%, #f8fbff 100%);\n      overflow: hidden;\n    }\n\n    #ols-d-orbitals-card #olsDOrbitalsViewer {\n      width: 100%;\n      height: 100%;\n      display: block;\n      cursor: grab;\n    }\n\n    #ols-d-orbitals-card #olsDOrbitalsViewer:active {\n      cursor: grabbing;\n    }\n\n    #ols-d-orbitals-card .ols-d-controls {\n      display: flex;\n      flex-wrap: wrap;\n      gap: 12px;\n      align-items: center;\n      justify-content: center;\n      padding: 18px 20px;\n      background: #ffffff;\n      border-top: 1px solid rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-d-orbitals-card .ols-d-btn {\n      appearance: none;\n      border: 1px solid transparent;\n      border-radius: 999px;\n      padding: 11px 18px;\n      font-family: Poppins, Arial, sans-serif;\n      font-size: 14px;\n      font-weight: 700;\n      color: #ffffff;\n      background: #1C244B;\n      box-shadow: 0 8px 18px rgba(28, 36, 75, 0.18);\n      cursor: pointer;\n      transition: transform 0.2s ease, box-shadow 0.2s ease, background 0.2s ease, color 0.2s ease, border-color 0.2s ease, opacity 0.2s ease;\n    }\n\n    #ols-d-orbitals-card .ols-d-btn:hover {\n      transform: translateY(-1px);\n      box-shadow: 0 12px 24px rgba(28, 36, 75, 0.22);\n      background: #27315e;\n    }\n\n    #ols-d-orbitals-card .ols-d-btn.secondary {\n      color: #1C244B;\n      background: #EEF4FF;\n      border: 1px solid rgba(28, 36, 75, 0.12);\n      box-shadow: none;\n    }\n\n    #ols-d-orbitals-card .ols-d-btn.secondary:hover {\n      background: #e4edff;\n      box-shadow: none;\n      transform: translateY(-1px);\n    }\n\n    #ols-d-orbitals-card .ols-d-orbital-btn {\n      box-shadow: none;\n    }\n\n    #ols-d-orbitals-card .ols-d-orbital-btn:hover {\n      box-shadow: none;\n    }\n\n    #ols-d-orbitals-card .ols-d-orbital-btn[data-orbital=\"dxy\"] {\n      background: #c81e1e;\n      border-color: #c81e1e;\n      color: #ffffff;\n    }\n\n    #ols-d-orbitals-card .ols-d-orbital-btn[data-orbital=\"dxz\"] {\n      background: #7a3db8;\n      border-color: #7a3db8;\n      color: #ffffff;\n    }\n\n    #ols-d-orbitals-card .ols-d-orbital-btn[data-orbital=\"dyz\"] {\n      background: #2f8f46;\n      border-color: #2f8f46;\n      color: #ffffff;\n    }\n\n    #ols-d-orbitals-card .ols-d-orbital-btn[data-orbital=\"dx2y2\"] {\n      background: #c9973a;\n      border-color: #c9973a;\n      color: #ffffff;\n    }\n\n    #ols-d-orbitals-card .ols-d-orbital-btn[data-orbital=\"dz2\"] {\n      background: #2166c2;\n      border-color: #2166c2;\n      color: #ffffff;\n    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   border-color: rgba(33, 102, 194, 0.30);\n    }\n\n    #ols-d-orbitals-card .ols-d-info-grid {\n      display: grid;\n      grid-template-columns: repeat(4, minmax(0, 1fr));\n      gap: 16px;\n      padding: 22px;\n      background: #f8fbff;\n      border-top: 1px solid rgba(28, 36, 75, 0.08);\n    }\n\n    #ols-d-orbitals-card .ols-d-info-box {\n      background: #ffffff;\n      border: 1px solid rgba(28, 36, 75, 0.10);\n      border-radius: 20px;\n      padding: 18px 18px 16px;\n      box-shadow: 0 8px 22px rgba(28, 36, 75, 0.06);\n    }\n\n    #ols-d-orbitals-card .ols-d-info-box h3 {\n      margin: 0 0 8px;\n      font-size: 17px;\n      line-height: 1.25;\n      font-weight: 700;\n      color: #1C244B;\n    }\n\n    #ols-d-orbitals-card .ols-d-info-box p {\n      margin: 0;\n      font-size: 15px;\n      line-height: 1.55;\n      font-weight: 300;\n      color: rgba(28, 36, 75, 0.82);\n    }\n\n    #ols-d-orbitals-card .ols-key-red {\n      color: #c81e1e;\n      font-weight: 700;\n    }\n\n    #ols-d-orbitals-card .ols-key-purple {\n      color: #7a3db8;\n      font-weight: 700;\n    }\n\n    #ols-d-orbitals-card .ols-key-green {\n      color: #2f8f46;\n      font-weight: 700;\n    }\n\n    #ols-d-orbitals-card .ols-key-gold {\n      color: #c9973a;\n      font-weight: 700;\n    }\n\n    #ols-d-orbitals-card .ols-key-blue {\n      color: #2166c2;\n      font-weight: 700;\n    }\n\n    @media (max-width: 1000px) {\n      #ols-d-orbitals-card .ols-d-info-grid {\n        grid-template-columns: repeat(2, minmax(0, 1fr));\n      }\n    }\n\n    @media (max-width: 900px) {\n      #ols-d-orbitals-card .ols-d-viewer-wrap {\n        height: 500px;\n      }\n    }\n\n    @media (max-width: 600px) {\n      #ols-d-orbitals-card .ols-d-header {\n        padding: 24px 20px 16px;\n      }\n\n      #ols-d-orbitals-card .ols-d-subtitle {\n        font-size: 16px;\n      }\n\n      #ols-d-orbitals-card .ols-d-viewer-wrap {\n        height: 430px;\n      }\n\n      #ols-d-orbitals-card .ols-d-controls {\n        justify-content: stretch;\n      }\n\n      #ols-d-orbitals-card .ols-d-btn {\n        width: 100%;\n      }\n\n      #ols-d-orbitals-card .ols-d-info-grid {\n        grid-template-columns: 1fr;\n      }\n    }\n  <\/style>\n\n  <div class=\"ols-d-card\">\n    <div class=\"ols-d-header\">\n      <div class=\"ols-d-pill\">Atomic orbitals \u2022 d orbitals<\/div>\n      <h2 class=\"ols-d-title\">The Five d Orbitals<\/h2>\n      <p class=\"ols-d-subtitle\">\n        This 3D model lets you explore the five d orbitals one at a time. Use the colour-coded buttons to switch between <span class=\"ols-key-red\">d<sub>xy<\/sub><\/span>, <span class=\"ols-key-purple\">d<sub>xz<\/sub><\/span>, <span class=\"ols-key-green\">d<sub>yz<\/sub><\/span>, <span class=\"ols-key-gold\">d<sub>x\u00b2\u2212y\u00b2<\/sub><\/span>, and <span class=\"ols-key-blue\">d<sub>z\u00b2<\/sub><\/span>. The Cartesian axes are centred at the origin so the orientation of each orbital is visually clear.\n      <\/p>\n    <\/div>\n\n    <div class=\"ols-d-viewer-wrap\">\n      <div id=\"olsDOrbitalsViewer\"><\/div>\n    <\/div>\n\n    <div class=\"ols-d-controls\">\n      <button class=\"ols-d-btn\" id=\"olsDOrbitalsToggleSpin\" type=\"button\">Pause rotation<\/button>\n      <button class=\"ols-d-btn secondary\" id=\"olsDOrbitalsResetView\" type=\"button\">Reset view<\/button>\n      <button class=\"ols-d-btn ols-d-orbital-btn\" id=\"olsSelectDxy\" data-orbital=\"dxy\" type=\"button\" aria-pressed=\"true\">d<sub>xy<\/sub><\/button>\n      <button class=\"ols-d-btn ols-d-orbital-btn inactive\" id=\"olsSelectDxz\" data-orbital=\"dxz\" type=\"button\" aria-pressed=\"false\">d<sub>xz<\/sub><\/button>\n      <button class=\"ols-d-btn ols-d-orbital-btn inactive\" id=\"olsSelectDyz\" data-orbital=\"dyz\" type=\"button\" aria-pressed=\"false\">d<sub>yz<\/sub><\/button>\n      <button class=\"ols-d-btn ols-d-orbital-btn inactive\" id=\"olsSelectDx2y2\" data-orbital=\"dx2y2\" type=\"button\" aria-pressed=\"false\">d<sub>x\u00b2\u2212y\u00b2<\/sub><\/button>\n      <button class=\"ols-d-btn ols-d-orbital-btn inactive\" id=\"olsSelectDz2\" data-orbital=\"dz2\" type=\"button\" aria-pressed=\"false\">d<sub>z\u00b2<\/sub><\/button>\n    <\/div>\n\n    <div class=\"ols-d-info-grid\">\n      <div class=\"ols-d-info-box\">\n        <h3>Selected orbital<\/h3>\n        <p id=\"olsDSelectedOrbitalText\">\n          <strong>d<sub>xy<\/sub><\/strong> has four lobes lying between the x- and y-axes in the xy plane.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-d-info-box\">\n        <h3>Four-lobed orbitals<\/h3>\n        <p>\n          The <strong>d<sub>xy<\/sub><\/strong>, <strong>d<sub>xz<\/sub><\/strong>, <strong>d<sub>yz<\/sub><\/strong>, and <strong>d<sub>x\u00b2\u2212y\u00b2<\/sub><\/strong> orbitals each have a four-lobed cloverleaf shape.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-d-info-box\">\n        <h3>d<sub>z\u00b2<\/sub> shape<\/h3>\n        <p>\n          The <strong>d<sub>z\u00b2<\/sub><\/strong> orbital is different. It has two lobes along the z-axis and a torus-shaped ring around the centre.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-d-info-box\">\n        <h3>Key idea<\/h3>\n        <p>\n          The fixed Cartesian axes help show whether the lobes point along the axes or between them.\n        <\/p>\n      <\/div>\n    <\/div>\n  <\/div>\n\n  <script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/d-orbitals.js\"><\/script>\n  <p style=\"text-align:center;font-size:11px;color:#aab0c0;font-style:italic;margin:0;padding:10px 0;\">\u00a9 Dr. Mohammed Al-Fatah \u00b7 <a href=\"https:\/\/onlinelearningsystem.net\" style=\"color:#aab0c0;text-decoration:none;\">onlinelearningsystem.net<\/a><\/p>\n<\/section>\n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">4<\/div><h2>Writing Electronic Configuration<\/h2><\/div>\n        <p>An atom&#8217;s <strong>electronic configuration<\/strong> shows how its electrons are arranged within atomic orbitals. The notation combines the main energy level, the subshell type and the number of electrons in that subshell.<\/p>\n        <p>For example, oxygen is written as <strong>1s\u00b2 2s\u00b2 2p\u2074<\/strong>. The large number shows the main energy level, the letter shows the subshell type, and the superscript shows the number of electrons in that subshell.<\/p>\n        <p>Electrons fill subshells in order of increasing energy. For the early part of the sequence, the order is <strong>1s \u2192 2s \u2192 2p \u2192 3s \u2192 3p \u2192 4s \u2192 3d \u2192 4p<\/strong>.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Exam focus:<\/strong> The 4s subshell fills before 3d because 4s is lower in energy than 3d before the 3d subshell is occupied.<\/p><\/div>\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\">\n            <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Atomic-electron-configuration-infographic.webp\" alt=\"Electronic configuration notation showing energy level, subshell and number of electrons\">\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <p>This notation shows the shell number, subshell letter and the number of electrons present in that subshell.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">5<\/div><h2>Electronic Configuration of Ions<\/h2><\/div>\n        <p>When ions form, electrons are either lost or gained. A <strong>positive ion<\/strong> forms when electrons are lost. A <strong>negative ion<\/strong> forms when electrons are gained.<\/p>\n        <p>For magnesium, the atom is written as <strong>1s\u00b2 2s\u00b2 2p\u2076 3s\u00b2<\/strong>, but Mg\u00b2\u207a is written as <strong>1s\u00b2 2s\u00b2 2p\u2076<\/strong> because two electrons have been lost.<\/p>\n        <p>For oxygen, the atom is written as <strong>1s\u00b2 2s\u00b2 2p\u2074<\/strong>, but O\u00b2\u207b is written as <strong>1s\u00b2 2s\u00b2 2p\u2076<\/strong> because two electrons have been gained.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Remember:<\/strong> Ion formation changes the number of electrons, not the number of protons.<\/p><\/div>\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\">\n            <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Understanding-electronic-structure-and-ions.webp\" alt=\"Electronic configurations of magnesium and oxygen ions\">\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <p>Positive ions have fewer electrons than their atoms. Negative ions have more electrons than their atoms.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">6<\/div><h2>Hund&#8217;s Rule<\/h2><\/div>\n        <p><strong>Hund&#8217;s rule<\/strong> states that electrons occupy orbitals singly before any pairing occurs in orbitals of the same energy.<\/p>\n        <p>This minimises electron-electron repulsion. In a p subshell, there are three p orbitals, so one electron enters each p orbital before any two electrons pair up in the same orbital.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Exam focus:<\/strong> When filling p orbitals, show one arrow in each box first, then pair electrons only after each equal-energy orbital contains one electron.<\/p><\/div>\n      <\/article>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">7<\/div><h2>Pauli Exclusion Principle and Spin Diagrams<\/h2><\/div>\n        <p>The <strong>Pauli exclusion principle<\/strong> states that no two electrons in the same orbital can have the same spin. This means paired electrons in one orbital must be shown with <strong>opposite spins<\/strong>.<\/p>\n        <p>In spin diagrams, each box represents one orbital, and each arrow represents one electron. Opposite arrow directions represent opposite electron spins.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Remember:<\/strong> One orbital can hold a maximum of two electrons, and those two electrons must have opposite spin.<\/p><\/div>\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\">\n            <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Using-spin-diagrams-for-fluorine.webp\" alt=\"Spin diagram for fluorine showing paired and unpaired electrons\">\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <p>Each box is one orbital. Arrows in opposite directions show paired electrons with opposite spin.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">8<\/div><h2>Common Exam Points<\/h2><\/div>\n        <p>Quantum model questions often test precise language about orbitals, correct electronic configurations, and correct use of Hund&#8217;s rule and the Pauli exclusion principle.<\/p>\n        <div class=\"ols-rule-list\">\n          <div class=\"ols-rule-item\"><h3>Define an orbital accurately<\/h3><p>An orbital is a region of space where an electron is most likely to be found. It can hold a maximum of two electrons.<\/p><\/div>\n          <div class=\"ols-rule-item\"><h3>Use the correct filling order<\/h3><p>Remember that 4s fills before 3d in the usual Aufbau sequence for neutral atoms.<\/p><\/div>\n          <div class=\"ols-rule-item\"><h3>Apply Hund&#8217;s rule<\/h3><p>Place electrons singly in equal-energy orbitals before pairing them.<\/p><\/div>\n          <div class=\"ols-rule-item\"><h3>Apply Pauli correctly<\/h3><p>Two electrons in the same orbital must have opposite spins.<\/p><\/div>\n        <\/div>\n      <\/article>\n\n      <section class=\"ols-h5p-card\">\n        <h2>Check Your Understanding<\/h2>\n        <p>Use these short activities to check shells, subshells, orbitals, electronic configuration, Hund&#8217;s rule and the Pauli exclusion principle before moving on.<\/p>\n        <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-259\" class=\"h5p-iframe\" data-content-id=\"259\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"T2.3.1 Drag: Quantum Model Key Concepts\"><\/iframe><\/div><\/div>\n        <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-260\" class=\"h5p-iframe\" data-content-id=\"260\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"T2.3.2 MCQ: Definition of an Orbital\"><\/iframe><\/div><\/div>\n        <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-261\" class=\"h5p-iframe\" data-content-id=\"261\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"T2.3.3 MCQ: Electronic Configuration of Calcium\"><\/iframe><\/div><\/div>\n        <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-344\" class=\"h5p-iframe\" data-content-id=\"344\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Electronic Configuration: Arrow and Box Method\"><\/iframe><\/div><\/div>\n        <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-345\" class=\"h5p-iframe\" data-content-id=\"345\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Electronic Configuration: Orbital Method\"><\/iframe><\/div><\/div>\n        <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-346\" class=\"h5p-iframe\" data-content-id=\"346\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Electronic Configuration: Shorthand Arrow and Box Method\"><\/iframe><\/div><\/div>\n        <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-347\" class=\"h5p-iframe\" data-content-id=\"347\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Electronic Configuration: Shorthand Orbital Method\"><\/iframe><\/div><\/div>\n        <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-262\" class=\"h5p-iframe\" data-content-id=\"262\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"T2.3.4 MCQ: Hunds Rule and Pauli Principle\"><\/iframe><\/div><\/div>\n        <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-306\" class=\"h5p-iframe\" data-content-id=\"306\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Atom with the most orbitals containing a single electron\"><\/iframe><\/div><\/div>\n      <\/section>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">\u2713<\/div><h2>QuickSnap<\/h2><\/div>\n        <p>The quantum model describes electrons using probability-based orbitals rather than fixed paths. A shell is a main energy level, a subshell is a subdivision of a shell, and an orbital is a region where an electron is most likely to be found. The s orbital is spherical, p orbitals are dumbbell-shaped, and d orbitals have more complex shapes. Electronic configurations show how electrons fill subshells in order of increasing energy. Hund&#8217;s rule says electrons occupy equal-energy orbitals singly before pairing, while the Pauli exclusion principle says two electrons in the same orbital must have opposite spins.<\/p>\n        <div class=\"ols-rule-list\">\n          <div class=\"ols-rule-item\"><h3>Shell<\/h3><p>A principal energy level defined by the quantum number n.<\/p><\/div>\n          <div class=\"ols-rule-item\"><h3>Subshell<\/h3><p>A subdivision of a shell, such as s, p, d or f.<\/p><\/div>\n          <div class=\"ols-rule-item\"><h3>Orbital<\/h3><p>A region of high electron probability that can hold up to two electrons.<\/p><\/div>\n          <div class=\"ols-rule-item\"><h3>Spin diagrams<\/h3><p>Boxes represent orbitals, arrows represent electrons, and opposite arrows show opposite spin.<\/p><\/div>\n        <\/div>\n      <\/article>\n\n<section class=\"ols-course-cta-atomic-structure\" id=\"olsCourseCtaAtomicStructure002\">\n  <style>\n    .ols-course-cta-atomic-structure,\n    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Course\n          <\/div>\n\n          <a class=\"ols-course-button ols-course-button-top\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/product\/atomic-structure-and-periodic-table-edexcel-t2\/\">\n            View Course\n          <\/a>\n\n        <\/div>\n\n        <h2>\n          Master Atomic Structure and The Periodic Table for Edexcel International A Level Chemistry\n        <\/h2>\n\n      <\/div>\n\n    <\/div>\n\n    <div class=\"ols-course-cta-intro-stats\">\n\n      <p class=\"ols-course-cta-intro\">\n        Continue from these free revision notes into the full Topic 2 Atomic Structure and The Periodic Table course, with guided video teaching, diagnostic MCQ practice, teacher-marked short-answer questions and a specification assignment with a personalised progress report.\n      <\/p>\n\n      <div class=\"ols-course-cta-stats\">\n\n        <div class=\"ols-course-stat\">\n          <span>Guided learning<\/span>\n          <strong>18 hours<\/strong>\n        <\/div>\n\n        <div class=\"ols-course-stat\">\n          <span>Video lessons<\/span>\n          <strong>369 mins<\/strong>\n        <\/div>\n\n        <div class=\"ols-course-stat\">\n          <span>MCQ practice<\/span>\n          <strong>86 marks<\/strong>\n        <\/div>\n\n        <div class=\"ols-course-stat\">\n          <span>SAQ practice<\/span>\n          <strong>215 marks<\/strong>\n        <\/div>\n\n      <\/div>\n\n    <\/div>\n\n    <div class=\"ols-course-cta-features\">\n\n      <div class=\"ols-course-feature\">\n        <h3>Guided video teaching<\/h3>\n        <p>\n          Learn atomic structure, isotopes, mass spectrometry, ionisation energy, electron configuration and periodic trends through structured video lessons with worked examples and walkthroughs.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-course-feature\">\n        <h3>Instant MCQ feedback<\/h3>\n        <p>\n          Auto-marked MCQ quizzes provide immediate diagnostic feedback for 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<\/h1>\n        <\/div>\n      <\/div>\n\n      <section id=\"videoScene\" class=\"ols-video-scene\">\n        <div class=\"ols-video-scale-wrap\">\n          <div class=\"ols-video-card\">\n            <video id=\"lessonVideo\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Virtual-Lesson.mp4\" muted=\"\" playsinline=\"\" preload=\"auto\">\n            <\/video>\n          <\/div>\n        <\/div>\n      <\/section>\n    <\/div>\n  <\/section>\n\n  <section id=\"sceneMcq\" class=\"ols-scene\">\n    <div class=\"ols-mcq-stage\">\n      <div id=\"mcqIntroScreen\" class=\"ols-mcq-intro-screen\">\n        <div class=\"ols-mcq-intro-title-wrap\">\n          <h1 class=\"ols-mcq-intro-title\">\n            <span id=\"mcqIntroTitleText\"><\/span><span id=\"mcqIntroTitleCursor\" class=\"ols-title-cursor\"><\/span>\n          <\/h1>\n        <\/div>\n      <\/div>\n\n      <div class=\"ols-mcq-scale-wrap\">\n        <main id=\"mcqScreen\" class=\"ols-mcq-screen\">\n          <section class=\"mcq-question-area\">\n            <p class=\"mcq-question-lead\">Some successive ionisation energies for element X are shown.<\/p>\n\n            <table class=\"mcq-data-table\">\n              <thead>\n                <tr>\n                  <th>Ionisation energy<\/th>\n                  <th>Value \/ kJ mol<sup>\u22121<\/sup><\/th>\n                <\/tr>\n              <\/thead>\n              <tbody>\n                <tr><td>1st<\/td><td>738<\/td><\/tr>\n                <tr><td>2nd<\/td><td>1451<\/td><\/tr>\n                <tr><td>3rd<\/td><td>7733<\/td><\/tr>\n                <tr><td>4th<\/td><td>10543<\/td><\/tr>\n              <\/tbody>\n            <\/table>\n\n            <p class=\"mcq-question-main\">Which formula is most likely for the fluoride of element X?<\/p>\n\n            <div class=\"mcq-options-wrap\">\n              <div id=\"mcqWrongOption\" class=\"mcq-option-row\">\n                <span class=\"mcq-radio\"><\/span>\n                <span class=\"mcq-radio-ripple\"><\/span>\n                <span class=\"mcq-option-label\">XF<\/span>\n\n                <span id=\"mcqSpecificFeedback\" class=\"mcq-specific-feedback\">\n                  <span id=\"mcqCrossIcon\" class=\"mcq-cross-icon\">\u00d7<\/span>\n                  <span class=\"mcq-specific-feedback-text\">\n                    <span id=\"mcqSpecificText\"><\/span><span id=\"mcqSpecificCursor\" class=\"cursor mcq-specific-cursor\" style=\"display:none;\"><\/span>\n                  <\/span>\n                <\/span>\n              <\/div>\n\n              <div class=\"mcq-option-row\"><span class=\"mcq-radio\"><\/span><span class=\"mcq-radio-ripple\"><\/span><span class=\"mcq-option-label\">XF<sub>2<\/sub><\/span><\/div>\n              <div class=\"mcq-option-row\"><span class=\"mcq-radio\"><\/span><span class=\"mcq-radio-ripple\"><\/span><span class=\"mcq-option-label\">XF<sub>3<\/sub><\/span><\/div>\n            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Each orbital can hold a maximum of two electrons.<\/p><\/div>\n          <div class=\"ols-faq-item\"><h3>What is the difference between a shell and a subshell?<\/h3><p>A shell is a principal energy level. A subshell is a subdivision within a shell, such as s, p, d or f.<\/p><\/div>\n          <div class=\"ols-faq-item\"><h3>What shape is an s orbital?<\/h3><p>An s orbital is spherical, so it has the same shape in all directions around the nucleus.<\/p><\/div>\n          <div class=\"ols-faq-item\"><h3>What shape are p orbitals?<\/h3><p>p orbitals are dumbbell-shaped. The three p orbitals, p<sub>x<\/sub>, p<sub>y<\/sub> and p<sub>z<\/sub>, are arranged at right angles to each other.<\/p><\/div>\n          <div class=\"ols-faq-item\"><h3>What does Hund's rule state?<\/h3><p>Hund's rule states that electrons occupy orbitals singly before any pairing occurs in orbitals of the same energy.<\/p><\/div>\n          <div class=\"ols-faq-item\"><h3>What does the Pauli exclusion principle state?<\/h3><p>No two electrons in the same orbital can have the same spin. Two electrons in the same orbital must have opposite spins.<\/p><\/div>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-related-card\">\n        <h2>Related Topics<\/h2>\n        <p>Continue with the linked Topic 2 revision pages to connect the quantum model with atomic structure, mass spectrometry and ionisation energy.<\/p>\n        <div class=\"ols-related-grid\">\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/atomic-structure\/\">Atomic Structure<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/mass-spectrometry\/\">Mass Spectrometry<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/\">Ionisation Energy<\/a>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-attribution-card\">\n        <p><strong>Copyright notice:<\/strong> This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. 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