{"id":4513,"date":"2026-05-28T06:03:42","date_gmt":"2026-05-28T05:03:42","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-1-atomic-structure-and-the-periodic-table\/atomic-structure\/"},"modified":"2026-10-25T12:13:35","modified_gmt":"2026-10-25T12:13:35","slug":"atomic-structure","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-1-atomic-structure-and-the-periodic-table\/atomic-structure\/","title":{"rendered":"Atomic Structure"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-atomic-structure-9ch0-page\">\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      --soft-green: #f0f7f1;\n      --soft-orange: #fff7ed;\n      --grey-text: #667085;\n      --body-text: #1f2937;\n      --border: rgba(28, 36, 75, 0.14);\n      --shadow: 0 18px 45px rgba(28, 36, 75, 0.10);\n      --inner-shadow: 0 10px 26px rgba(28, 36, 75, 0.08);\n      font-family: Poppins, Arial, sans-serif;\n      color: var(--navy);\n      background: #ffffff;\n    }\n\n    .ols-revision-page * { box-sizing: border-box; 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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      .ols-left-numbers { font-size: 26px; }\n      .ols-symbol-core { font-size: 52px; }\n      .ols-symbol-labels { font-size: 13px; }\n    }\n  <\/style>\n\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    });\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>\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\/\">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>Atomic Structure<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Atomic Structure<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to subatomic particles, atomic number, mass number, nuclide notation, isotopes, relative isotopic mass and relative formula mass for Edexcel A Level Chemistry.<\/p>\n\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: isotopes-chlorine (23 Oct 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-iso-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-iso-001 a:hover{text-decoration:underline;}\n.ols-iso-001 .iso-seg button{display:inline-flex; align-items:center;}\n.ols-iso-001 .iso-seg button .iso-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-iso-001 .iso-seg button[aria-pressed=\"true\"] .iso-n{background:#ffffff; color:#2563eb;}\n<\/style>\n\n<div class=\"iso-head\">\n  <h2 class=\"iso-title\">Isotopes of Chlorine and Its Mass Spectrum<\/h2>\n  <p class=\"iso-sub\">Choose a step to build the two isotopes of chlorine particle by particle, sort a natural sample, then read its mass spectrum to find the relative atomic mass and the peaks of Cl\u2082.<\/p>\n<\/div>\n\n<div class=\"iso-stage\" id=\"isoStage\">\n  <canvas class=\"iso-canvas\" id=\"isoCanvas\"><\/canvas>\n  <div class=\"iso-overlay\" id=\"isoOverlay\"><\/div>\n  <div class=\"iso-caption\" id=\"isoCaption\"><\/div>\n  <div class=\"iso-hint\" id=\"isoHint\">Drag to rotate<\/div>\n  <div class=\"iso-badges\" id=\"isoBadges\"><\/div>\n<\/div>\n\n<div class=\"iso-controls\">\n  <div class=\"iso-row\">\n    <span class=\"iso-rowlab\">Step<\/span>\n    <div class=\"iso-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\"><span class=\"iso-n\">1<\/span>Chlorine-35<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\"><span class=\"iso-n\">2<\/span>Chlorine-37<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\"><span class=\"iso-n\">3<\/span>A natural sample<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\"><span class=\"iso-n\">4<\/span>Mass spectrum<\/button>\n      <button type=\"button\" data-step=\"5\" aria-pressed=\"false\"><span class=\"iso-n\">5<\/span>Relative atomic mass<\/button>\n      <button type=\"button\" data-step=\"6\" aria-pressed=\"false\"><span class=\"iso-n\">6<\/span>The Cl\u2082 molecule<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"iso-row\">\n    <span class=\"iso-rowlab\">View<\/span>\n    <button type=\"button\" class=\"iso-pill\" id=\"isoResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"iso-info\">\n  <h3 id=\"isoInfoTitle\"><\/h3>\n  <p id=\"isoInfoText\"><\/p>\n  <div class=\"iso-facts\" id=\"isoFacts\"><\/div>\n  <div class=\"iso-key\">\n    <span><i class=\"k-ball\" style=\"background:#cd2d28\"><\/i>Proton, p\u207a<\/span>\n    <span><i class=\"k-ball\" style=\"background:#969ca8\"><\/i>Neutron, n<\/span>\n    <span><i class=\"k-ball\" style=\"background:#f6c426\"><\/i>Electron<\/span>\n    <span><i class=\"k-ball\" style=\"background:#34aa48\"><\/i>\u00b3\u2075Cl atom<\/span>\n    <span><i class=\"k-ball\" style=\"background:#8054c4\"><\/i>\u00b3\u2077Cl atom<\/span>\n  <\/div>\n<\/div>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/isotopes-chlorine.js?v=20261025p\"><\/script>\n<\/section>\n\n<p class=\"ols-cc-iso-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\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: Proton Number and Isotopes<\/h2>\n<p>Before you start, check what you remember from GCSE about protons and relative atomic mass.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"304\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">1<\/div>\n          <h2>Subatomic Particles<\/h2>\n        <\/div>\n\n        <p>Atoms are made from three main subatomic particles: <strong>protons<\/strong>, <strong>neutrons<\/strong> and <strong>electrons<\/strong>.<\/p>\n        <p>Protons and neutrons are found in the <strong>nucleus<\/strong>, while electrons occupy regions of space around the nucleus. Protons and neutrons are also called <strong>nucleons<\/strong>.<\/p>\n        <p>Almost all of the mass of an atom is concentrated in the nucleus. The rest of the atom is mostly empty space.<\/p>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Particle<\/th>\n                <th>Position<\/th>\n                <th>Relative mass<\/th>\n                <th>Relative charge<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Particle\">Proton<\/td>\n                <td data-label=\"Position\">Nucleus<\/td>\n                <td data-label=\"Relative mass\">1<\/td>\n                <td data-label=\"Relative charge\">+1<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Particle\">Neutron<\/td>\n                <td data-label=\"Position\">Nucleus<\/td>\n                <td data-label=\"Relative mass\">1<\/td>\n                <td data-label=\"Relative charge\">0<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Particle\">Electron<\/td>\n                <td data-label=\"Position\">Orbitals or shells around the nucleus<\/td>\n                <td data-label=\"Relative mass\">1\/1840<\/td>\n                <td data-label=\"Relative charge\">-1<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n\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\/09\/wave-1.jpg\" alt=\"Bohr model of an atom: nucleus with 2, 8 and 1 electrons in three labelled shells, a KEY IDEAS panel and a nucleus\/electron key\">\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <p>This simplified Bohr model shows the nucleus at the centre with electrons arranged in shells around it.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">2<\/div>\n          <h2>Atomic Number and Mass Number<\/h2>\n        <\/div>\n\n        <p>The <strong>atomic number<\/strong>, symbol <strong>Z<\/strong>, is the number of protons in the nucleus of an atom.<\/p>\n        <p>The <strong>mass number<\/strong>, symbol <strong>A<\/strong>, is the total number of protons and neutrons in the atom.<\/p>\n        <p>For a neutral atom, the number of electrons is equal to the number of protons.<\/p>\n<p>An <strong>ion<\/strong> does not have equal numbers of protons and electrons.<\/p>\n<p>A positive ion has lost electrons and a negative ion has gained electrons, so number of electrons = number of protons &#8211; charge on the ion.<\/p>\n<p>The numbers of protons and neutrons do not change when an ion forms.<\/p>\n\n        <div class=\"ols-notation-card\">\n          <div class=\"ols-nuclide-symbol\" aria-label=\"Lithium nuclide notation showing mass number 7 and atomic number 3\">\n            <div class=\"ols-left-numbers\"><span>7<\/span><span>3<\/span><\/div>\n            <div class=\"ols-symbol-core\">Li<\/div>\n            <div class=\"ols-symbol-labels\"><span>mass number, A<\/span><span>atomic number, Z<\/span><\/div>\n          <\/div>\n          <div class=\"ols-formula-strip\">number of neutrons = A &#8211; Z<\/div>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Example:<\/strong> In <strong><sup>7<\/sup><sub>3<\/sub>Li<\/strong>, the atom has 3 protons and 7 &#8211; 3 = 4 neutrons. A neutral lithium atom also has 3 electrons.<\/p>\n        <\/div>\n<div class=\"ols-key-box\">\n<p><strong>Ion example:<\/strong> In <strong><sup>24<\/sup><sub>12<\/sub>Mg<sup>2+<\/sup><\/strong>, there are 12 protons, 24 &#8211; 12 = 12 neutrons and 12 &#8211; 2 = 10 electrons.<\/p>\n<p>In <strong><sup>19<\/sup><sub>9<\/sub>F<sup>\u2212<\/sup><\/strong>, there are 9 protons, 19 &#8211; 9 = 10 neutrons and 9 + 1 = 10 electrons.<\/p>\n<p>Both ions have 10 electrons, so they are described as <strong>isoelectronic<\/strong>.<\/p>\n<\/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: Count the Particles<\/h2>\n<p>Count the protons, neutrons and electrons in atoms and ions that are not on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-336\" class=\"h5p-iframe\" data-content-id=\"336\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Atomic Structure Quick-Fire: Counting Protons, Neutrons and Electrons\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">3<\/div>\n          <h2>Isotopes<\/h2>\n        <\/div>\n\n        <p><strong>Isotopes<\/strong> are atoms with the same number of protons but different numbers of neutrons.<\/p>\n        <p>Because isotopes of the same element have the same number of protons, they have the same atomic number. Their mass numbers are different because the number of neutrons is different.<\/p>\n        <p>Isotopes normally have very similar chemical properties because they have the same electronic structure. They may have slightly different physical properties because their masses are different.<\/p>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Isotope<\/th>\n                <th>Symbol<\/th>\n                <th>Number of protons<\/th>\n                <th>Number of neutrons<\/th>\n                <th>Number of electrons in a neutral atom<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Isotope\">chlorine-35<\/td>\n                <td data-label=\"Symbol\"><sup>35<\/sup><sub>17<\/sub>Cl<\/td>\n                <td data-label=\"Number of protons\">17<\/td>\n                <td data-label=\"Number of neutrons\">18<\/td>\n                <td data-label=\"Number of electrons in a neutral atom\">17<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Isotope\">chlorine-37<\/td>\n                <td data-label=\"Symbol\"><sup>37<\/sup><sub>17<\/sub>Cl<\/td>\n                <td data-label=\"Number of protons\">17<\/td>\n                <td data-label=\"Number of neutrons\">20<\/td>\n                <td data-label=\"Number of electrons in a neutral atom\">17<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Exam focus:<\/strong> Isotopes differ in neutrons, not protons. If the number of protons changes, it is a different element.<\/p>\n        <\/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: Write the Full Symbol<\/h2>\n<p>Write each full symbol on paper, then flip the card to check.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-337\" class=\"h5p-iframe\" data-content-id=\"337\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Atomic Structure: Write Isotope and Ion Symbols\"><\/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: Explain the Properties of an Isotope<\/h2>\n<p>Explain the difference between two kinds of water in a short written answer.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-235\" class=\"h5p-iframe\" data-content-id=\"235\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Isotopes Explain: Heavy Water and Ordinary Water\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">4<\/div>\n          <h2>Relative Isotopic Mass<\/h2>\n        <\/div>\n\n        <p><strong>Relative isotopic mass<\/strong> is the mass of one atom of an isotope compared with one twelfth of the mass of one atom of carbon-12.<\/p>\n        <p>Carbon-12 is used as the standard because one atom of carbon-12 is defined as exactly 12 atomic mass units.<\/p>\n        <p>This relative scale avoids using extremely small masses in kilograms when comparing atoms and isotopes.<\/p>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Term<\/th>\n                <th>Meaning<\/th>\n                <th>Exam wording to use<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Term\">Atomic mass unit<\/td>\n                <td data-label=\"Meaning\">One twelfth of the mass of one atom of carbon-12.<\/td>\n                <td data-label=\"Exam wording to use\">Use <strong>1\/12<\/strong> and <strong>carbon-12<\/strong>.<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Term\">Relative isotopic mass<\/td>\n                <td data-label=\"Meaning\">The mass of one atom of an isotope on the carbon-12 scale.<\/td>\n                <td data-label=\"Exam wording to use\">Refer to <strong>one atom<\/strong> of an isotope.<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Term\">Relative atomic mass<\/td>\n                <td data-label=\"Meaning\">The weighted mean mass of atoms of an element compared with 1\/12 of carbon-12.<\/td>\n                <td data-label=\"Exam wording to use\">This is covered in more depth in RAM calculations.<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/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: Using the Carbon-12 Scale<\/h2>\n<p>Use the carbon-12 scale to work out each relative isotopic mass.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-579\" class=\"h5p-iframe\" data-content-id=\"579\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Relative Isotopic Mass Fill in the Blanks: Using the Carbon-12 Scale\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">5<\/div>\n          <h2>Relative Molecular Mass and Relative Formula Mass<\/h2>\n        <\/div>\n\n        <p>The <strong>relative formula mass<\/strong>, M<sub>r<\/sub>, is found by adding together the relative atomic masses of all the atoms shown in a formula.<\/p>\n        <p>The term <strong>relative molecular mass<\/strong>, sometimes written RMM, should only be used for substances that exist as molecules.<\/p>\n<p><strong>Relative formula mass<\/strong> is the safer term because it applies to molecular substances, ionic compounds and giant structures. The specification states that the term <strong>relative formula mass<\/strong> should be used for compounds with giant structures.<\/p>\n\n        <div class=\"ols-definition-box\"><p><strong>Relative formula mass, M<sub>r<\/sub>:<\/strong> the sum of the relative atomic masses of all atoms in the formula of a substance.<\/p><\/div>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Substance<\/th>\n                <th>Formula<\/th>\n                <th>Calculation<\/th>\n                <th>M<sub>r<\/sub><\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Substance\">Carbon dioxide<\/td>\n                <td data-label=\"Formula\">CO<sub>2<\/sub><\/td>\n                <td data-label=\"Calculation\">12.0 + (2 \u00d7 16.0)<\/td>\n                <td data-label=\"Mr\">44.0<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Substance\">Calcium carbonate<\/td>\n                <td data-label=\"Formula\">CaCO<sub>3<\/sub><\/td>\n                <td data-label=\"Calculation\">40.1 + 12.0 + (3 \u00d7 16.0)<\/td>\n                <td data-label=\"Mr\">100.1<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Substance\">Magnesium nitrate<\/td>\n                <td data-label=\"Formula\">Mg(NO<sub>3<\/sub>)<sub>2<\/sub><\/td>\n                <td data-label=\"Calculation\">24.3 + 2(14.0 + 3 \u00d7 16.0)<\/td>\n                <td data-label=\"Mr\">148.3<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n\n        <div class=\"ols-key-box\"><p><strong>Exam focus:<\/strong> brackets multiply everything inside them. In Mg(NO<sub>3<\/sub>)<sub>2<\/sub>, there are two nitrogen atoms and six oxygen atoms.<\/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: Relative Formula Mass with Brackets<\/h2>\n<p>Work out each relative formula mass on paper, then type it.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-580\" class=\"h5p-iframe\" data-content-id=\"580\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Relative Formula Mass Flashcards: Formulae with Brackets\"><\/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: Atomic Structure Round-Up<\/h2>\n<p>Pick the one fully correct statement in each round to pull the whole page together.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1303\" class=\"h5p-iframe\" data-content-id=\"1303\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Atomic Structure Summary: Particles, Isotopes and Relative Masses\"><\/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\/10\/t1-img-07-formulamass.jpg\" alt=\"Worked relative formula masses: MgCO\u2083 84.3, Ca(OH)\u2082 74.1 and CuSO\u2084\u00b75H\u2082O 249.6, showing brackets and water of crystallisation\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n        <\/div>\n        <div class=\"ols-zoom-card-caption\">\n          <p>Relative formula mass is calculated by counting each atom in the formula and adding the correct relative atomic masses.<\/p>\n        <\/div>\n      <\/div>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">6<\/div>\n          <h2>Common Exam Points<\/h2>\n        <\/div>\n\n        <p>Atomic structure questions often reward precise definitions and careful counting. The most important terms are <strong>atomic number<\/strong>, <strong>mass number<\/strong>, <strong>isotope<\/strong>, <strong>neutron number<\/strong> and <strong>relative isotopic mass<\/strong>.<\/p>\n\n        <div class=\"ols-rule-list\">\n          <div class=\"ols-rule-item\">\n            <h3>Do not confuse atomic number and mass number<\/h3>\n            <p>Atomic number is the number of protons. Mass number is the total number of protons and neutrons.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>Use A &#8211; Z for neutrons<\/h3>\n            <p>The number of neutrons is found by subtracting the atomic number from the mass number.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>Neutral atoms have equal protons and electrons<\/h3>\n            <p>If the atom is neutral, the positive charge from protons is balanced by the negative charge from electrons. In an ion the numbers are not equal: a 2+ ion has two fewer electrons than protons, and a 2\u2212 ion has two more.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>Isotopes have the same electronic structure<\/h3>\n            <p>This explains why isotopes of the same element have similar chemical properties.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n\n      \n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">\u2713<\/div>\n          <h2>QuickSnap<\/h2>\n        <\/div>\n\n        <p>For this page, the essential idea is that the nucleus contains protons and neutrons, while electrons occupy regions around the nucleus. Atomic number counts protons, mass number counts protons plus neutrons, and isotopes have the same number of protons but different numbers of neutrons.<\/p>\n\n        <div class=\"ols-rule-list\">\n          <div class=\"ols-rule-item\">\n            <h3>Subatomic particles<\/h3>\n            <p>Protons have charge +1, neutrons have charge 0, and electrons have charge -1. Protons and neutrons have relative mass 1, while electrons have a much smaller relative mass of about 1\/1840.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>Atomic number and mass number<\/h3>\n            <p>Atomic number, Z, gives the number of protons. Mass number, A, gives the number of protons plus neutrons.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>Isotopes<\/h3>\n            <p>Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>Carbon-12 scale<\/h3>\n            <p>Relative isotopic mass compares one atom of an isotope with one twelfth of the mass of one atom of carbon-12.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n\n      \n\n      <section class=\"ols-faq-card\">\n        <h2>Atomic Structure FAQs<\/h2>\n        <p>Use these quick answers to check the definitions most commonly assessed in early atomic structure questions.<\/p>\n\n        <div class=\"ols-faq-list\">\n          <div class=\"ols-faq-item\">\n            <h3>What is the atomic number?<\/h3>\n            <p>The atomic number is the number of protons in the nucleus of an atom. It is also called Z.<\/p>\n          <\/div>\n\n          <div class=\"ols-faq-item\">\n            <h3>What is the mass number?<\/h3>\n            <p>The mass number is the total number of protons and neutrons in the nucleus of an atom. It is also called A.<\/p>\n          <\/div>\n\n          <div class=\"ols-faq-item\">\n            <h3>How do you calculate the number of neutrons?<\/h3>\n            <p>Use number of neutrons = mass number &#8211; atomic number. In symbols, this is A &#8211; Z.<\/p>\n          <\/div>\n\n          <div class=\"ols-faq-item\">\n            <h3>What are isotopes?<\/h3>\n            <p>Isotopes are atoms with the same number of protons but different numbers of neutrons.<\/p>\n          <\/div>\n\n          <div class=\"ols-faq-item\">\n            <h3>Why do isotopes have similar chemical properties?<\/h3>\n            <p>Isotopes of the same element have the same number of electrons and the same electronic structure, so they normally react in similar ways.<\/p>\n          <\/div>\n        <\/div>\n      <\/section>\n\n      <!-- author card (moved to the end, 10 Oct 2026) -->\n      <div class=\"ols-author\">\n        \n            <img decoding=\"async\"\n              class=\"ols-author-avatar-img\"\n              src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\"\n              alt=\"Dr. Mohammed Al-Fatah\"\n            >\n        \n            <div class=\"ols-author-content\">\n        \n              <h2 class=\"ols-author-title\">\n                Written by: Dr. Mohammed Al-Fatah\n              <\/h2>\n        \n              <p class=\"ols-author-description\">\n                Chemistry specialist revision notes for A Level Chemistry.\n              <\/p>\n        \n              <a\n                class=\"ols-linkedin-pill\"\n                href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\"\n                target=\"_blank\"\n                rel=\"noopener noreferrer\"\n              >\n        \n                <svg class=\"ols-linkedin-icon\" viewBox=\"0 0 24 24\" fill=\"currentColor\" aria-hidden=\"true\">\n                  <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\"\/>\n                <\/svg>\n        \n                View LinkedIn Profile\n        \n              <\/a>\n        \n            <\/div>\n        \n          <\/div>\n\n\n      <section class=\"ols-related-card\">\n        <h2>Related Atomic Structure Revision Notes<\/h2>\n        <p>Continue through Topic 1 by linking atomic structure to mass spectrometry, electron configuration and ionisation energy.<\/p>\n\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\/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\/electron-configuration\/\">Electron Configuration<\/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      <!-- OLS Author Copyright Footprint \/ Attribution Card -->\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<script>\n        (function(){\n          var page = document.querySelector(\".ols-atomic-structure-9ch0-page\");\n          if (!page) {\n            return;\n          }\n\n          var lightbox = page.querySelector(\"#olsImageLightboxAtomicStructure9ch0\");\n          if (!lightbox) {\n            return;\n          }\n\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\n            if (!document.body.dataset.olsPreviousOverflow) {\n              document.body.dataset.olsPreviousOverflow = document.body.style.overflow || \"default\";\n            }\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\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(event){\n              event.preventDefault();\n              event.stopPropagation();\n              openLightbox(image);\n            });\n          });\n\n          closeButton.addEventListener(\"click\", closeLightbox);\n\n          lightbox.addEventListener(\"click\", function(event){\n            if (event.target === lightbox) {\n              closeLightbox();\n            }\n          });\n\n          document.addEventListener(\"keydown\", function(event){\n            if (event.key === \"Escape\" && lightbox.classList.contains(\"is-open\")) {\n              closeLightbox();\n            }\n          });\n        })();\n      <\/script>\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\/atomic-structure\/#webpage\",\n            \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-1-atomic-structure-and-the-periodic-table\/atomic-structure\/\",\n            \"name\": \"Atomic Structure - 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