{"id":4515,"date":"2026-05-28T06:03:48","date_gmt":"2026-05-28T05:03:48","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-1-atomic-structure-and-the-periodic-table\/electron-configuration\/"},"modified":"2026-10-25T12:07:08","modified_gmt":"2026-10-25T12:07:08","slug":"electron-configuration","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-1-atomic-structure-and-the-periodic-table\/electron-configuration\/","title":{"rendered":"Electron Configuration"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-electron-configuration-9ch0-page\">\n<script src=\"https:\/\/cdnjs.cloudflare.com\/ajax\/libs\/three.js\/r128\/three.min.js\"><\/script>\n  <style>\n    .ols-revision-page {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --soft-blue: #eef4ff;\n      --soft-red: #fff7f7;\n      --soft-purple: #f7f0ff;\n      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clamp(20px, 2.2vw, 27px); letter-spacing: -0.02em; }\n    .ols-h5p-card.ols-h5p-inline > p { margin: 8px 0 0; }\n    .ols-h5p-card.ols-h5p-inline .ols-h5p-frame { margin-top: 16px; padding: 14px; border-radius: 20px; }\n    .ols-h5p-kicker { display: inline-block; margin-bottom: 10px; padding: 5px 12px; border-radius: 999px; background: #ede9fe; color: #5b21b6; font-size: 12px; font-weight: 700; letter-spacing: 0.06em; text-transform: uppercase; }\n    .ols-h5p-card.ols-h5p-recap { border-left-color: #c9973a; background: linear-gradient(135deg, #ffffff 0%, #fff8e8 100%); }\n    .ols-h5p-recap .ols-h5p-kicker { background: #fdf0d2; color: #8a5a00; }\n    @media (max-width: 760px) { .ols-h5p-card.ols-h5p-inline { padding: 20px 16px; } }\n<\/style>\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 1 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\/topic-1-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\/topic-1-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\/topic-1-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\/topic-1-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\/topic-1-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\/topic-1-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\/topic-1-atomic-structure-and-the-periodic-table\/electron-configuration\/\">Electron Configuration<\/a>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-1-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\/topic-1-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\/topic-1-atomic-structure-and-the-periodic-table\/ionisation-energy\/successive-ionisation-energies\/\">Successive Ionisation Energies<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-1-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\/topic-1-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\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-1-atomic-structure-and-the-periodic-table\/periodic-trends\/\">Periodic Trends<\/a>\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\/topic-2-bonding-and-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\/topic-3-redox-i\/\">Redox I<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-4-inorganic-chemistry-and-the-periodic-table\/\">Inorganic Chemistry &amp; The Periodic Table<\/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    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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\/\">Edexcel<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-1-atomic-structure-and-the-periodic-table\/\">Topic 1 Atomic Structure &amp; The Periodic Table<\/a> \/\n        <span>Electron Configuration<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Electron Configuration<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to quantum shells, subshells, s and p orbital shapes, electronic configuration notation, Hund&#8217;s rule and the Pauli exclusion principle for Edexcel A Level Chemistry (9CH0).<\/p>\n        <div class=\"ols-badges\">\n          <div class=\"ols-badge\">Unit: Paper 1<\/div>\n          <div class=\"ols-badge\">Topic 1: Atomic Structure &amp; The Periodic Table<\/div>\n          <div class=\"ols-badge\">9CH0\/01<\/div>\n        <\/div>\n      <\/header>\n<!-- 3D card: atomic-orbitals (swapped 26 Sep 2026) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-aorb-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-aorb-001 a:hover{text-decoration:underline;}\n.ols-aorb-001 .aorb-seg button{display:inline-flex; align-items:center;}\n.ols-aorb-001 .aorb-seg button .aorb-n{display:inline-flex; align-items:center; justify-content:center; width:20px; height:20px; margin-right:9px; border-radius:50%; font-size:11px; font-weight:600; background:rgba(37,99,235,0.10); color:#2563eb; flex:0 0 auto;}\n.ols-aorb-001 .aorb-seg button[aria-pressed=\"true\"] .aorb-n{background:#ffffff; color:#2563eb;}\n<\/style>\n\n<div class=\"aorb-head\">\n  <h2 class=\"aorb-title\">Shapes of Atomic Orbitals: s, p and d<\/h2>\n  <p class=\"aorb-sub\">Choose a step to watch each type of orbital form round a nucleus on fixed x, y and z axes, then build up a full second shell.<\/p>\n<\/div>\n\n<div class=\"aorb-stage\" id=\"aorbStage\">\n  <canvas class=\"aorb-canvas\" id=\"aorbCanvas\"><\/canvas>\n  <div class=\"aorb-overlay\" id=\"aorbOverlay\"><\/div>\n  <div class=\"aorb-caption\" id=\"aorbCaption\"><\/div>\n  <div class=\"aorb-hint\" id=\"aorbHint\">Drag to rotate<\/div>\n  <div class=\"aorb-badges\" id=\"aorbBadges\"><\/div>\n<\/div>\n\n<div class=\"aorb-controls\">\n  <div class=\"aorb-row\">\n    <span class=\"aorb-rowlab\">Step<\/span>\n    <div class=\"aorb-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\"><span class=\"aorb-n\">1<\/span>s orbital<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\"><span class=\"aorb-n\">2<\/span>p<sub>x<\/sub> orbital<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\"><span class=\"aorb-n\">3<\/span>Three p orbitals<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\"><span class=\"aorb-n\">4<\/span>Five d orbitals<\/button>\n      <button type=\"button\" data-step=\"5\" aria-pressed=\"false\"><span class=\"aorb-n\">5<\/span>Filling the second shell<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"aorb-row\">\n    <span class=\"aorb-rowlab\">View<\/span>\n    <button type=\"button\" class=\"aorb-pill\" id=\"aorbResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"aorb-info\">\n  <h3 id=\"aorbInfoTitle\"><\/h3>\n  <p id=\"aorbInfoText\"><\/p>\n  <div class=\"aorb-facts\" id=\"aorbFacts\"><\/div>\n  <div class=\"aorb-key\">\n    <span><i class=\"k-ball\" style=\"background:#484e5c\"><\/i>Nucleus<\/span>\n    <span><i class=\"\" style=\"background:rgba(150,156,170,0.6)\"><\/i>s orbital<\/span>\n    <span><i class=\"\" style=\"background:rgba(236,140,40,0.65)\"><\/i>p<sub>x<\/sub><\/span>\n    <span><i class=\"\" style=\"background:rgba(140,90,200,0.65)\"><\/i>p<sub>y<\/sub><\/span>\n    <span><i class=\"\" style=\"background:rgba(60,165,90,0.65)\"><\/i>p<sub>z<\/sub><\/span>\n    <span><i class=\"\" style=\"background:rgba(30,146,160,0.65)\"><\/i>d orbital<\/span>\n    <span><i class=\"\" style=\"background:rgba(59,120,220,0.14);border-color:rgba(40,86,170,0.7)\"><\/i>Nodal plane<\/span>\n  <\/div>\n<\/div>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/atomic-orbitals.js?v=20261025p\"><\/script>\n<\/section>\n<!-- 3D card: electron-filling (23 Oct 2026) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. All rights reserved. -->\n<section class=\"ols-efl-001\" id=\"olsEfl001\">\n<style>\n@import url('https:\/\/fonts.googleapis.com\/css2?family=Poppins:wght@300;400;500;600&display=swap');\n\n.ols-efl-001{\n  font-family:'Poppins',system-ui,-apple-system,'Segoe UI',Roboto,Helvetica,Arial,sans-serif;\n  color:#1C244B; background:#ffffff;\n  border:1px solid rgba(28,36,75,0.14);\n  border-radius:28px;\n  box-shadow:0 18px 45px rgba(28,36,75,0.10);\n  padding:44px; margin:26px auto; max-width:980px;\n  box-sizing:border-box; -webkit-font-smoothing:antialiased;\n}\n.ols-efl-001 *{box-sizing:border-box;}\n\n.ols-efl-001 .efl-head{margin:0 0 22px;}\n.ols-efl-001 h2.efl-title{\n  font-size:26px; line-height:1.25; font-weight:600; letter-spacing:-0.012em;\n  margin:0 0 9px; color:#1C244B;\n}\n.ols-efl-001 p.efl-sub{\n  font-size:14.5px; line-height:1.62; font-weight:300; color:#4a5270;\n  margin:0; max-width:66ch;\n}\n\n.ols-efl-001 .efl-stage{\n  position:relative; width:100%; height:540px;\n  background:#ffffff; 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color:#b02a24;}\n.ols-efl-001 .efl-chip.efl-short{font-size:13px; color:#2f5f9a;}\n.ols-efl-001 .efl-chip:empty{display:none;}\n.ols-efl-001 .efl-key i.k-up{\n  width:auto; height:auto; border:0; background:none; font-style:normal; font-weight:600; color:#c98a12; margin-right:6px;\n}\n@media (max-width:760px){\n  .ols-efl-001 .efl-tile{width:48px; height:48px; border-radius:10px;}\n  .ols-efl-001 .efl-tile .efl-sym{font-size:17px; margin-top:5px;}\n  .ols-efl-001 .efl-tile .efl-z{font-size:9px; left:5px; top:3px;}\n  .ols-efl-001 .efl-meta .efl-nm{font-size:10.5px;}\n  .ols-efl-001 .efl-meta .efl-ec{font-size:10px;}\n  .ols-efl-001 .efl-chip{font-size:11.5px; padding:5px 9px;}\n  .ols-efl-001 .efl-chip.efl-short{font-size:11px;}\n  .ols-efl-001 .efl-hud{gap:4px;}\n}\n@media (prefers-reduced-motion:reduce){\n  .ols-efl-001 .efl-hud{transition:none;}\n}\n<\/style>\n\n<div class=\"efl-head\">\n  <h2 class=\"efl-title\">Filling Orbitals: Electrons in Boxes<\/h2>\n  <p class=\"efl-sub\">Choose a step to watch electrons fill the orbitals in order of energy, from hydrogen to zinc, then see which electrons leave first when iron forms its ions.<\/p>\n<\/div>\n\n<div class=\"efl-stage\" id=\"eflStage\">\n  <canvas class=\"efl-canvas\" id=\"eflCanvas\"><\/canvas>\n  <div class=\"efl-overlay\" id=\"eflOverlay\"><\/div>\n  <div class=\"efl-caption\" id=\"eflCaption\"><\/div>\n  <div class=\"efl-hint\" id=\"eflHint\">Drag to rotate<\/div>\n  <div class=\"efl-hud\" id=\"eflHud\">\n    <div class=\"efl-tilerow\">\n      <div class=\"efl-meta\"><span class=\"efl-nm\" id=\"eflNm\"><\/span><span class=\"efl-ec\" id=\"eflEc\"><\/span><\/div>\n      <div class=\"efl-tile\"><span class=\"efl-z\" id=\"eflZ\"><\/span><b class=\"efl-sym\" id=\"eflSym\"><\/b><\/div>\n    <\/div>\n    <div class=\"efl-chip\" id=\"eflChip\"><\/div>\n    <div class=\"efl-chip efl-short\" id=\"eflShort\"><\/div>\n  <\/div>\n  <div class=\"efl-badges\" id=\"eflBadges\"><\/div>\n<\/div>\n\n<div class=\"efl-controls\">\n  <div class=\"efl-row\">\n    <span class=\"efl-rowlab\">Step<\/span>\n    <div class=\"efl-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\"><span class=\"efl-n\">1<\/span>Energy levels<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\"><span class=\"efl-n\">2<\/span>H to Ne<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\"><span class=\"efl-n\">3<\/span>Na to Ca<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\"><span class=\"efl-n\">4<\/span>The d block<\/button>\n      <button type=\"button\" data-step=\"5\" aria-pressed=\"false\"><span class=\"efl-n\">5<\/span>Ions<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"efl-row\">\n    <span class=\"efl-rowlab\">View<\/span>\n    <button type=\"button\" class=\"efl-pill\" id=\"eflResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"efl-info\">\n  <h3 id=\"eflInfoTitle\"><\/h3>\n  <p id=\"eflInfoText\"><\/p>\n  <div class=\"efl-facts\" id=\"eflFacts\"><\/div>\n  <div class=\"efl-key\">\n    <span><i style=\"background:#c8ccd6\"><\/i>s orbital<\/span>\n    <span><i style=\"background:#e88c30\"><\/i>p<sub>x<\/sub><\/span>\n    <span><i style=\"background:#8c5cc8\"><\/i>p<sub>y<\/sub><\/span>\n    <span><i style=\"background:#1c9ea0\"><\/i>p<sub>z<\/sub><\/span>\n    <span><i style=\"background:#648ed4\"><\/i>d orbital<\/span>\n    <span><i class=\"k-up\">\u2191\u2193<\/i>electrons with opposite spins<\/span>\n    <span><i style=\"background:#ffffff; border-style:dashed\"><\/i>empty orbital<\/span>\n  <\/div>\n<\/div>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/electron-filling.js?v=20261025p\"><\/script>\r\n<\/section>\r\n\r\n<p class=\"ols-cc-efl-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n\n<p class=\"ols-cc-aorb-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n\n      <section class=\"ols-h5p-card ols-h5p-inline ols-h5p-recap\">\n<span class=\"ols-h5p-kicker\">Before you start<\/span>\n<h2>GCSE Recap: Electron Shells 2,8,8<\/h2>\n<p>Before you start, check that you can still place electrons in shells the GCSE way.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"578\"><\/div><\/div>\n<\/section>\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        <p>Each type of subshell has a fixed number of orbitals. An <strong>s subshell<\/strong> has 1 orbital, a <strong>p subshell<\/strong> has 3 orbitals and a <strong>d subshell<\/strong> has 5 orbitals.<\/p>\n<p>Each orbital holds up to 2 electrons, so s, p and d subshells hold a maximum of <strong>2, 6 and 10 electrons<\/strong>.<\/p>\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<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Shells, Subshells and Orbitals<\/h2>\n<p>Answer three short questions on what an orbital is and how many electrons a subshell or shell can hold.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"259\"><\/div><\/div>\n<\/section>\n\n      \n\n      \n\n      \n\n      \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        <p>Long configurations can be shortened by using a <strong>noble gas core<\/strong>. The symbol [Ar] stands for the 18 electrons of argon, 1s\u00b2 2s\u00b2 2p\u2076 3s\u00b2 3p\u2076.<\/p>\n<p>Manganese (25 electrons) is 1s\u00b2 2s\u00b2 2p\u2076 3s\u00b2 3p\u2076 4s\u00b2 3d\u2075, which shortens to <strong>[Ar] 4s\u00b2 3d\u2075<\/strong>.<\/p>\n<p>Write the configuration in full when the question asks for the full electronic configuration. The 3d and 4s subshells can be written in either order, so [Ar] 3d\u2075 4s\u00b2 is also correct.<\/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      <\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Electrons in a Named Subshell<\/h2>\n<p>Write each configuration on paper, then type the number of electrons asked for.<\/p>\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=\"Electron Configuration Flashcards: Electrons in a Named Subshell\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <div class=\"ols-zoom-card\">\n        <div class=\"ols-zoom-card-image\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/fix-11.jpg\" alt=\"How to write an electronic configuration: the meaning of each part of 2p4, sub-shell capacities, and the 1s to 4p filling order.\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n        <\/div>\n        <div class=\"ols-zoom-card-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\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      <\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Ions with the Same Configuration<\/h2>\n<p>Count the electrons in each species and click every one that has the configuration shown.<\/p>\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=\"Electron Configuration Mark the Words: Species with the Neon Configuration\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\"><div class=\"ols-note-icon\">6<\/div><h2>Chromium, Copper and d-Block Ions<\/h2><\/div>\n<p>Two atoms in Period 4 do not follow the expected filling pattern. <strong>Chromium<\/strong> and <strong>copper<\/strong> each have only one electron in 4s.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Atom<\/th><th>Expected but wrong<\/th><th>Actual configuration<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td data-label=\"Atom\">Chromium (24 electrons)<\/td><td data-label=\"Expected but wrong\">[Ar] 4s\u00b2 3d\u2074<\/td><td data-label=\"Actual configuration\"><strong>[Ar] 4s\u00b9 3d\u2075<\/strong><\/td><\/tr>\n<tr><td data-label=\"Atom\">Copper (29 electrons)<\/td><td data-label=\"Expected but wrong\">[Ar] 4s\u00b2 3d\u2079<\/td><td data-label=\"Actual configuration\"><strong>[Ar] 4s\u00b9 3d\u00b9\u2070<\/strong><\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>A <strong>half-filled<\/strong> 3d subshell (3d\u2075) and a <strong>full<\/strong> 3d subshell (3d\u00b9\u2070) are particularly stable arrangements, so one electron occupies 3d instead of pairing up in 4s.<\/p>\n<p>When a d-block atom forms a positive ion, the <strong>4s electrons<\/strong> are lost before the 3d electrons. The 4s subshell fills first, but once 3d contains electrons, 4s is higher in energy, so it also empties first.<\/p>\n<p>For example, manganese is [Ar] 4s\u00b2 3d\u2075, so Mn\u00b2\u207a is <strong>[Ar] 3d\u2075<\/strong>, not [Ar] 4s\u00b2 3d\u00b3. Copper is [Ar] 4s\u00b9 3d\u00b9\u2070, so Cu\u00b2\u207a is <strong>[Ar] 3d\u2079<\/strong>.<\/p>\n<div class=\"ols-key-box\"><p><strong>Common mistake:<\/strong> Removing 3d electrons first when writing the configuration of a d-block ion. In the d-block, 4s is filled first and emptied first.<\/p><\/div>\n<\/article>\n\n      <div class=\"ols-zoom-card\">\n        <div class=\"ols-zoom-card-image\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t2-img-19-ionconfig.jpg\" alt=\"Electronic configurations of ions: Mg loses two 3s electrons to form Mg\u00b2\u207a and O gains two 2p electrons to form O\u00b2\u207b, both 1s\u00b2 2s\u00b2 2p\u2076\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n        <\/div>\n        <div class=\"ols-zoom-card-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\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">7<\/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\">8<\/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      <\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Judge the Box Diagrams<\/h2>\n<p>In each round, pick the one statement that is accurate.<\/p>\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=\"Electrons-in-Boxes Summary: Hund&#039;s Rule, Pauli Principle and Unpaired Electrons\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Draw Electrons-in-Boxes Diagrams<\/h2>\n<p>Draw each diagram on paper before you flip the card.<\/p>\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=\"Electrons-in-Boxes Flip Cards: Draw the Diagram\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <div class=\"ols-zoom-card\">\n        <div class=\"ols-zoom-card-image\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/wave-43.jpg\" alt=\"Electrons-in-boxes spin diagram for fluorine, 1s, 2s and 2p, with callouts for the arrow, the box and the opposite spins, plus a filling-rule panel\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n        <\/div>\n        <div class=\"ols-zoom-card-caption\">\n          <p>Each box is one orbital. Arrows in opposite directions show paired electrons with opposite spin.<\/p>\n        <\/div>\n      <\/div>\n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">9<\/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. The exceptions are chromium (4s\u00b9 3d\u2075) and copper (4s\u00b9 3d\u00b9\u2070). When d-block ions form, the 4s electrons are lost first.<\/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      \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.<\/p>\n<p>The s orbital is spherical, p orbitals are dumbbell-shaped, and d orbitals have more complex shapes.<\/p>\n<p>Electronic configurations show how electrons fill subshells in order of increasing energy.<\/p>\n<p>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\n\n      <section class=\"ols-faq-card\">\n        <h2>Electron Configuration FAQs<\/h2>\n        <p>Use these quick answers to secure the key wording for Edexcel A Level Chemistry.<\/p>\n        <div class=\"ols-faq-list\">\n          <div class=\"ols-faq-item\"><h3>What is an orbital?<\/h3><p>An orbital is a region of space where an electron is most likely to be found. 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&#8217;s rule state?<\/h3><p>Hund&#8217;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      <!-- author card (moved to the end, 10 Oct 2026) -->\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\n\n      <section class=\"ols-related-card\">\n        <h2>Related Topics<\/h2>\n        <p>Continue with the linked Topic 1 revision pages to connect electron configuration 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\/topic-1-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\/topic-1-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\/topic-1-atomic-structure-and-the-periodic-table\/ionisation-energy\/\">Ionisation Energy<\/a>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-attribution-card ols-attr-std\" aria-label=\"Copyright notice\">\n<style id=\"ols-attribution-card-css\">.ols-attribution-card.ols-attr-std{background:linear-gradient(135deg,#ffffff,#f8fbff);border:1px solid rgba(28,36,75,0.14);border-radius:28px;box-shadow:0 18px 45px rgba(28,36,75,0.10);padding:34px;margin:24px 0;overflow:hidden;font-family:Poppins,Arial,sans-serif;box-sizing:border-box}.ols-attribution-card.ols-attr-std *{box-sizing:border-box}.ols-attribution-card.ols-attr-std p{margin:0;font-size:14px;line-height:1.65;font-weight:300;color:#667085}.ols-attribution-card.ols-attr-std strong{font-weight:700;color:#1C244B}@media (max-width:760px){.ols-attribution-card.ols-attr-std{padding:24px 18px;border-radius:22px}}<\/style>\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. It may not be copied, reproduced, redistributed or adapted without written permission.<\/p>\n<\/section>\n\n      <div class=\"ols-image-lightbox\" id=\"olsImageLightboxElectronConfig9ch0\" aria-hidden=\"true\" role=\"dialog\" aria-modal=\"true\" aria-label=\"Expanded revision image\">\n        <div class=\"ols-image-lightbox-inner\">\n          <button class=\"ols-image-lightbox-close\" type=\"button\" aria-label=\"Close enlarged image\">\u00d7<\/button>\n          <img decoding=\"async\" class=\"ols-image-lightbox-img\" src=\"\" alt=\"\">\n        <\/div>\n      <\/div>\n\n      <script>\n        (function(){\n          var page = document.querySelector(\".ols-electron-configuration-9ch0-page\");\n          if (!page) return;\n          var lightbox = page.querySelector(\"#olsImageLightboxElectronConfig9ch0\");\n          if (!lightbox) return;\n          var lightboxImage = lightbox.querySelector(\".ols-image-lightbox-img\");\n          var closeButton = lightbox.querySelector(\".ols-image-lightbox-close\");\n          var clickableImages = page.querySelectorAll(\"img.ols-lightbox-target\");\n\n          function openLightbox(image) {\n            lightboxImage.src = image.currentSrc || image.src;\n            lightboxImage.alt = image.alt || \"Expanded revision image\";\n            lightbox.classList.add(\"is-open\");\n            lightbox.setAttribute(\"aria-hidden\", \"false\");\n            if (!document.body.dataset.olsPreviousOverflow) {\n              document.body.dataset.olsPreviousOverflow = document.body.style.overflow || \"default\";\n            }\n            document.body.style.overflow = \"hidden\";\n          }\n\n          function closeLightbox() {\n            lightbox.classList.remove(\"is-open\");\n            lightbox.setAttribute(\"aria-hidden\", \"true\");\n            lightboxImage.src = \"\";\n            if (document.body.dataset.olsPreviousOverflow) {\n              document.body.style.overflow = document.body.dataset.olsPreviousOverflow === \"default\" ? \"\" : document.body.dataset.olsPreviousOverflow;\n              delete document.body.dataset.olsPreviousOverflow;\n            }\n          }\n\n          clickableImages.forEach(function(image){\n            image.addEventListener(\"click\", function(e){\n              e.preventDefault();\n              e.stopPropagation();\n              openLightbox(image);\n            });\n          });\n\n          closeButton.addEventListener(\"click\", closeLightbox);\n          lightbox.addEventListener(\"click\", function(e){ if (e.target === lightbox) closeLightbox(); });\n          document.addEventListener(\"keydown\", function(e){ if (e.key === \"Escape\" && lightbox.classList.contains(\"is-open\")) closeLightbox(); });\n        })();\n      <\/script>\n    <\/main>\n  \n\n  <script type=\"application\/ld+json\">\n{\n    \"@context\": \"https:\/\/schema.org\",\n    \"@graph\": [\n        {\n            \"@type\": \"WebPage\",\n            \"@id\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-1-atomic-structure-and-the-periodic-table\/electron-configuration\/#webpage\",\n            \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-1-atomic-structure-and-the-periodic-table\/electron-configuration\/\",\n            \"name\": \"Electron Configuration - 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