{"id":4522,"date":"2026-05-28T06:05:21","date_gmt":"2026-05-28T05:05:21","guid":{"rendered":"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\/"},"modified":"2026-10-14T09:05:22","modified_gmt":"2026-10-14T08:05:22","slug":"introduction-to-ionisation-energy","status":"publish","type":"page","link":"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\/","title":{"rendered":"Introduction to Ionisation Energy"},"content":{"rendered":"\n<!--\n===============================================================================\nONLINE LEARNING SYSTEM COPYRIGHT NOTICE\n\u00a9 Online Learning System. 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     }\n\n      .ols-note-title {\n        align-items: flex-start;\n      }\n\n      .ols-related-grid,\n      .ols-h5p-grid {\n        grid-template-columns: 1fr;\n      }\n\n      .ols-table-wrap {\n        border: none;\n        background: transparent;\n      }\n\n      .ols-table,\n      .ols-table thead,\n      .ols-table tbody,\n      .ols-table th,\n      .ols-table td,\n      .ols-table tr {\n        display: block;\n        width: 100%;\n      }\n\n      .ols-table {\n        border-collapse: separate;\n        border-spacing: 0;\n      }\n\n      .ols-table thead {\n        display: none;\n      }\n\n      .ols-table tr {\n        margin-bottom: 14px;\n        border: 1px solid var(--border);\n        border-radius: 18px;\n        overflow: hidden;\n        background: #ffffff;\n        box-shadow: var(--inner-shadow);\n      }\n\n      .ols-table td {\n        border-bottom: 1px solid var(--border);\n        padding: 14px 16px;\n        overflow-wrap: anywhere;\n      }\n\n      .ols-table td:last-child {\n        border-bottom: none;\n      }\n\n      .ols-table td::before {\n        content: attr(data-label);\n        display: block;\n        margin-bottom: 6px;\n        font-size: 13px;\n        font-weight: 700;\n        color: var(--blue);\n      }\n\n      .ols-author {\n        align-items: flex-start;\n        padding: 22px 18px;\n        border-radius: 22px;\n      }\n\n      .ols-author-avatar-img {\n        width: 72px;\n        height: 72px;\n      }\n\n      .ols-author-title {\n        font-size: 24px;\n      }\n\n      .ols-author-description {\n        font-size: 15px;\n      }\n    }\n  <\/style>\n\n  <div class=\"ols-revision-layout\">\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  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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        <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> \/\n        <span>Introduction to Ionisation Energy<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Introduction to Ionisation Energy<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to first ionisation energy, second ionisation energy and the three main factors that affect how strongly an outer electron is attracted to the nucleus.<\/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\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: Forming Positive Ions<\/h2>\n<p>Before you start, check that you remember how atoms become positive ions.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"589\"><\/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>What Is First Ionisation Energy?<\/h2><\/div>\n        <p><strong>First ionisation energy<\/strong> is the energy required when <strong>one mole of gaseous atoms<\/strong> forms <strong>one mole of gaseous ions with a single positive charge<\/strong>.<\/p>\n        <p>The electron is removed from each gaseous atom. Because an electron is removed, a positive ion forms.<\/p>\n        <p>The atoms must be <strong>gaseous<\/strong> so that they are separate from each other.<\/p>\n<p>There are then no bonds or forces between particles to overcome, so the energy measured is only the energy needed to remove the electron.<\/p>\n<p>This is why the equation for oxygen starts from O(g) atoms, not O\u2082(g) molecules.<\/p>\n        <p>The value is quoted for <strong>one mole<\/strong> of atoms, in kJ mol\u207b\u00b9. This gives a convenient number and lets different elements be compared on the same basis.<\/p>\n\n        <div class=\"ols-equation-box\">\n          O(g) \u2192 O\u207a(g) + e\u207b\n          <small>For oxygen, the first ionisation energy is +1314 kJ mol\u207b\u00b9.<\/small>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Key idea:<\/strong> The definition must include one mole, gaseous atoms, gaseous ions and a single positive charge.<\/p>\n        <\/div>\n      <div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t2-img-20-ie1definition.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t2-img-20-ie1definition.jpg\" alt=\"The first ionisation energy equation for oxygen with call-outs for each part of the definition, and the second ionisation equation below\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t2-img-20-ie1definition.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Each part of the equation matches a part of the definition: one mole of gaseous atoms forming one mole of gaseous 1+ ions.<\/p><\/div>\n<\/div>\n<\/article>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">2<\/div><h2>Second Ionisation Energy<\/h2><\/div>\n        <p><strong>Second ionisation energy<\/strong> is the energy required when <strong>one mole of gaseous ions with a single positive charge<\/strong> forms <strong>one mole of gaseous ions with a double positive charge<\/strong>.<\/p>\n        <p>This means the second electron is removed from an ion that is already positive. This normally requires more energy than removing the first electron because the remaining electrons are attracted more strongly to the positively charged species.<\/p>\n\n        <div class=\"ols-equation-box\">\n          O\u207a(g) \u2192 O\u00b2\u207a(g) + e\u207b\n          <small>This represents the second ionisation of oxygen ions.<\/small>\n        <\/div>\n\n        <p>Third and later ionisation energies follow the same pattern. The <strong>nth ionisation energy<\/strong> removes one electron from each ion in one mole of gaseous ions with a charge of (n \u2212 1)+.<\/p>\n\n        <div class=\"ols-equation-box\">\n          X<sup>(n\u22121)+<\/sup>(g) \u2192 X<sup>n+<\/sup>(g) + e<sup>\u2212<\/sup>\n          <small>General equation for the nth ionisation energy. The third ionisation energy of oxygen is O\u00b2\u207a(g) \u2192 O\u00b3\u207a(g) + e\u207b.<\/small>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Exam focus:<\/strong> Do not define second ionisation energy using neutral atoms. It starts with gaseous 1+ ions and forms gaseous 2+ ions.<\/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: Choose the Ionisation Equation<\/h2>\n<p>Pick the equation that matches each ionisation energy.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-265\" class=\"h5p-iframe\" data-content-id=\"265\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ionisation Energy Single Choice: First and Second Ionisation Equations\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">3<\/div><h2>Why Ionisation Energy Is Positive<\/h2><\/div>\n        <p>Ionisation energy is positive because energy must be supplied to overcome the attraction between the negatively charged electron and the positively charged nucleus.<\/p>\n        <p>A <strong>high ionisation energy<\/strong> indicates a <strong>strong attraction between the electron and the nucleus<\/strong>. As a result, more energy is needed to remove the electron from the atom or ion.<\/p>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Remember:<\/strong> Removing an electron requires energy input, so ionisation energies are endothermic and usually shown with a positive sign.<\/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: Moles and Signs<\/h2>\n<p>Use the two ionisation energies given to work out each number.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-590\" class=\"h5p-iframe\" data-content-id=\"590\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ionisation Energy Fill in the Blanks: Moles and Signs\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">4<\/div><h2>Three Factors That Affect Ionisation Energy<\/h2><\/div>\n        <p>Ionisation energy depends on how strongly the outer electron is attracted to the nucleus. Three factors are most important: <strong>nuclear charge<\/strong>, <strong>distance from the nucleus<\/strong> and <strong>shielding<\/strong>.<\/p>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Factor<\/th>\n                <th>Effect on attraction<\/th>\n                <th>Effect on ionisation energy<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Factor\"><strong>Nuclear charge<\/strong><\/td>\n                <td data-label=\"Effect on attraction\">More protons in the nucleus make the nucleus more positively charged, so attraction for outer electrons increases.<\/td>\n                <td data-label=\"Effect on ionisation energy\">Higher nuclear charge usually increases ionisation energy.<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Factor\"><strong>Distance from the nucleus<\/strong><\/td>\n                <td data-label=\"Effect on attraction\">Attraction falls rapidly with distance. An electron in a shell closer to the nucleus is more strongly attracted.<\/td>\n                <td data-label=\"Effect on ionisation energy\">Greater distance from the nucleus usually decreases ionisation energy.<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Factor\"><strong>Shielding<\/strong><\/td>\n                <td data-label=\"Effect on attraction\">Inner shells of electrons repel outer electrons and reduce the attraction from the nucleus.<\/td>\n                <td data-label=\"Effect on ionisation energy\">More shielding usually decreases ionisation energy.<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Key idea:<\/strong> Many ionisation energy questions can be answered by applying these three factors carefully.<\/p>\n        <\/div>\n      <div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t2-img-21-iefactors.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t2-img-21-iefactors.jpg\" alt=\"Three shell diagrams showing how nuclear charge, distance from the nucleus and shielding change ionisation energy, with helium against lithium\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t2-img-21-iefactors.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Nuclear charge raises ionisation energy; greater distance and more shielding lower it.<\/p><\/div>\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: Do Neutrons Matter?<\/h2>\n<p>Use the three factors to compare two isotopes of the same element.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-266\" class=\"h5p-iframe\" data-content-id=\"266\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ionisation Energy MCQ: Isotopes and the Three Factors\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">5<\/div><h2>How to Apply the Factors<\/h2><\/div>\n        <p>When comparing ionisation energies, start by asking which electron is being removed and how strongly it is attracted to the nucleus.<\/p>\n\n        <div class=\"ols-rule-list\">\n          <div class=\"ols-rule-item\">\n            <h3>More protons usually means stronger attraction<\/h3>\n            <p>If shielding and distance are similar, a higher nuclear charge increases attraction for the outer electron.<\/p>\n          <\/div>\n\n          <div class=\"ols-rule-item\">\n            <h3>More shells usually means weaker attraction<\/h3>\n            <p>If the outer electron is in a shell further from the nucleus, it experiences weaker attraction.<\/p>\n          <\/div>\n\n          <div class=\"ols-rule-item\">\n            <h3>More shielding lowers the effective nuclear attraction<\/h3>\n            <p>Inner electrons repel outer electrons, so the outer electron feels less of the full nuclear charge.<\/p>\n          <\/div>\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: Explain Lithium and Helium<\/h2>\n<p>Write a short explanation, then compare it with the mark points and the model answer.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-348\" class=\"h5p-iframe\" data-content-id=\"348\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ionisation Energy Explain: Lithium Compared with Helium\"><\/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: Pick the Accurate Statement<\/h2>\n<p>In each round, choose the one statement that is accurate.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-263\" class=\"h5p-iframe\" data-content-id=\"263\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ionisation Energy Summary: Definitions and Factors Applied\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">6<\/div><h2>Common Exam Points<\/h2><\/div>\n        <ul>\n          <li>Always state <strong>gaseous atoms<\/strong> or <strong>gaseous ions<\/strong> in ionisation energy definitions.<\/li>\n          <li>Always refer to <strong>one mole<\/strong>, not one atom, when giving the formal definition.<\/li>\n          <li>For first ionisation energy, the product is <strong>gaseous 1+ ions<\/strong>.<\/li>\n          <li>For second ionisation energy, the reactant is already a <strong>gaseous 1+ ion<\/strong>.<\/li>\n          <li>When explaining trends, use <strong>nuclear charge<\/strong>, <strong>distance from the nucleus<\/strong> and <strong>shielding<\/strong>.<\/li>\n          <li>Do not just say attraction is stronger or weaker. State why the attraction changes.<\/li>\n        <\/ul>\n      <\/article>\n\n      \n\n      <section class=\"ols-quicksnap-card\">\n        <h2>QuickSnap<\/h2>\n        <p>This text summary condenses the page into the essential exam ideas.<\/p>\n        <ul class=\"ols-quicksnap-list\">\n          <li><strong>First ionisation energy:<\/strong> energy required when one mole of gaseous atoms forms one mole of gaseous 1+ ions.<\/li>\n          <li><strong>Second ionisation energy:<\/strong> energy required when one mole of gaseous 1+ ions forms one mole of gaseous 2+ ions.<\/li>\n          <li><strong>Why gaseous:<\/strong> the atoms are separate, so no bonds or forces between particles are included in the energy.<\/li>\n          <li><strong>nth ionisation energy:<\/strong> X<sup>(n\u22121)+<\/sup>(g) \u2192 X<sup>n+<\/sup>(g) + e<sup>\u2212<\/sup>.<\/li>\n          <li><strong>High ionisation energy:<\/strong> strong attraction between the electron and the nucleus.<\/li>\n          <li><strong>Main factors:<\/strong> nuclear charge, distance of the outer electron from the nucleus and shielding by inner electron shells.<\/li>\n          <li><strong>Exam method:<\/strong> explain attraction, then link the strength of attraction to the energy needed to remove the electron.<\/li>\n        <\/ul>\n      <\/section>\n\n\n\n      <section class=\"ols-faq-card\">\n        <h2>FAQs: Introduction to Ionisation Energy<\/h2>\n        <p>These frequently asked questions target the wording and reasoning students commonly need for exam answers.<\/p>\n\n        <div class=\"ols-faq-list\">\n          <div class=\"ols-faq-item\">\n            <h3>What is first ionisation energy?<\/h3>\n            <p>First ionisation energy is the energy required when one mole of gaseous atoms forms one mole of gaseous ions with a single positive charge.<\/p>\n          <\/div>\n\n          <div class=\"ols-faq-item\">\n            <h3>Why must the atoms be gaseous?<\/h3>\n            <p>The definition uses gaseous atoms so the energy change refers only to removing electrons from separate atoms, not to overcoming forces between particles in a solid or liquid.<\/p>\n          <\/div>\n\n          <div class=\"ols-faq-item\">\n            <h3>Why is ionisation energy positive?<\/h3>\n            <p>Energy must be supplied to overcome the attraction between the negatively charged electron and the positively charged nucleus, so ionisation is endothermic.<\/p>\n          <\/div>\n\n          <div class=\"ols-faq-item\">\n            <h3>What are the three main factors affecting ionisation energy?<\/h3>\n            <p>The three main factors are nuclear charge, distance of the electron from the nucleus and shielding by inner electron shells.<\/p>\n          <\/div>\n\n          <div class=\"ols-faq-item\">\n            <h3>Why does shielding lower ionisation energy?<\/h3>\n            <p>Inner shell electrons repel outer electrons and reduce the attraction between the outer electron and the nucleus. Less attraction means less energy is needed to remove the electron.<\/p>\n          <\/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 through Topic 1 by linking ionisation energy to successive ionisation energies, periodic trends and electron configuration.<\/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\/ionisation-energy\/successive-ionisation-energies\/\">Successive Ionisation Energy<\/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\/trends-in-a-period\/\">Trends in a Period<\/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\/trends-in-a-group\/\">Trends in a Group<\/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\/mass-spectrometry\/\">Mass Spectrometry<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-2-bonding-and-structure\/\">Bonding &amp; Structure<\/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. 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