{"id":3996,"date":"2026-05-21T04:56:52","date_gmt":"2026-05-21T03:56:52","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/trends-in-a-period\/"},"modified":"2026-06-28T05:52:48","modified_gmt":"2026-06-28T04:52:48","slug":"trends-in-a-period","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/trends-in-a-period\/","title":{"rendered":"Trends in a Period"},"content":{"rendered":"\n<!--\n===============================================================================\nONLINE LEARNING SYSTEM COPYRIGHT NOTICE\n\u00a9 Online Learning System. 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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/mass-spectrometry\/\">Mass Spectrometry<\/a>\r\n\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/mass-spectrometry\/the-mass-spectrometer\/\">The Mass Spectrometer<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/mass-spectrometry\/ram-calculations\/\">RAM Calculations<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/mass-spectrometry\/predicting-diatomic-mass-spectra\/\">Predicting Diatomic Mass Spectra<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/mass-spectrometry\/identifying-compounds-from-mass-spectra\/\">Identifying Compounds from Mass Spectra<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/quantum-model\/\">Quantum Model<\/a>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/\">Ionisation Energy<\/a>\r\n\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/introduction-to-ionisation-energy\/\">Introduction to Ionisation Energy<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/successive-ionisation-energy\/\">Successive Ionisation Energy<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/trends-in-a-period\/\">Trends in a Period<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/trends-in-a-group\/\">Trends in a Group<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Next Topics<\/h4>\r\n\r\n    <ul class=\"ols-topic-list\">\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-3-bonding-structure\/\">Bonding &amp; Structure<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-4-organic-chemistry-and-alkanes\/\">Organic Chemistry &amp; Alkanes<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/\">Alkenes<\/a>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/div>\r\n<\/aside>\r\n\r\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n    return String(path || '')\r\n      .split('?')[0]\r\n      .split('#')[0]\r\n      .replace(\/\\\/+$\/, '')\r\n      .toLowerCase();\r\n  }\r\n\r\n  function highlightActive() {\r\n    var sidebar = document.querySelector('.ols-sidebar');\r\n    if (!sidebar) return false;\r\n\r\n    var currentPath = normalisePath(window.location.pathname);\r\n    var links = sidebar.querySelectorAll('a[href]');\r\n    var matched = null;\r\n    var matchedLength = 0;\r\n\r\n    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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/\">Topic 2 Atomic Structure &amp; The Periodic Table<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/\">Ionisation Energy<\/a> \/\n        <span>Trends in a Period<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Trends in a Period<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to atomic radius and ionisation energy trends across a period, including the key exceptions from Group 2 to Group 3 and Group 5 to Group 6.<\/p>\n\n        <div class=\"ols-badges\">\n          <div class=\"ols-badge\">Unit 1: Structure, Bonding and Introduction to Organic Chemistry<\/div>\n          <div class=\"ols-badge\">Topic 2: Atomic Structure &amp; The Periodic Table<\/div>\n          <div class=\"ols-badge\">WCH11\/01<\/div>\n        <\/div>\n\n        <div class=\"ols-author\">\n          <img decoding=\"async\" class=\"ols-author-avatar-img\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\" alt=\"Dr. Mohammed Al-Fatah\">\n          <div class=\"ols-author-content\">\n            <h2 class=\"ols-author-title\">Written by:<br><span>Dr. Mohammed Al-Fatah<\/span><\/h2>\n            <p class=\"ols-author-description\">Chemistry specialist revision notes for A Level Chemistry.<\/p>\n            <a class=\"ols-linkedin-pill\" href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\" target=\"_blank\" rel=\"noopener noreferrer\">\n              <svg class=\"ols-linkedin-icon\" viewBox=\"0 0 24 24\" fill=\"currentColor\" aria-hidden=\"true\"><path d=\"M4.98 3.5C4.98 4.88 3.86 6 2.48 6S0 4.88 0 3.5 1.12 1 2.48 1s2.5 1.12 2.5 2.5zM.5 8h4V24h-4V8zm7 0h3.8v2.2h.1c.5-.9 1.8-2.2 3.9-2.2 4.2 0 5 2.8 5 6.4V24h-4v-7.6c0-1.8 0-4.2-2.6-4.2s-3 2-3 4v7.8h-4V8z\"><\/path><\/svg>\n              View LinkedIn Profile\n            <\/a>\n          <\/div>\n        <\/div>\n      <\/header>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">1<\/div><h2>What Changes Across a Period?<\/h2><\/div>\n        <p>Across a period, the <strong>number of protons increases<\/strong>. This increases the nuclear charge and gives the nucleus a stronger attraction for the outer electrons.<\/p>\n        <p>At the same time, new electrons are added to the <strong>same main energy level<\/strong>. They are not being added to new inner shells, so the amount of shielding remains roughly constant across the period.<\/p>\n        <div class=\"ols-key-box\">\n          <p><strong>Core idea:<\/strong> across a period, increasing nuclear charge has a greater effect than the small increase in electron-electron repulsion.<\/p>\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>Atomic Radius Decreases Across a Period<\/h2><\/div>\n        <p><strong>Atomic radius decreases<\/strong> across a period because the nucleus becomes more positively charged as proton number increases.<\/p>\n        <p>The added electrons enter the same outer energy level, so there is no major increase in shielding from additional inner shells. The stronger nuclear attraction pulls the electron cloud closer to the nucleus.<\/p>\n\n        <div class=\"ols-rule-list\">\n          <div class=\"ols-rule-item\">\n            <h3>Increased nuclear charge<\/h3>\n            <p>More protons give a stronger positive charge in the nucleus, so electrons are pulled closer.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>No extra inner-shell shielding<\/h3>\n            <p>Electrons are added to the same main energy level, not to new inner shells.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>Smaller atomic radius<\/h3>\n            <p>The outer electrons are attracted more strongly, so the radius decreases across the period.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">3<\/div><h2>First Ionisation Energy Generally Increases Across a Period<\/h2><\/div>\n        <p>First ionisation energy generally <strong>increases across a period<\/strong>. The outer electron is harder to remove because the nucleus has more protons and therefore exerts a stronger attraction.<\/p>\n        <p>Although each extra electron is added to the same shell and increases repulsion slightly, the increase in nuclear charge is more important. This is why the overall trend is an increase.<\/p>\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\/05\/The-first-ionisation-energy-trends.webp\" alt=\"First ionisation energy trends across Period 2 and Period 3\">\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <p>The graph shows the overall rise in first ionisation energy across a period, with small dips caused by subshell and electron-pairing effects.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Exam wording:<\/strong> as proton number increases, shielding remains similar because electrons are added to the same shell, so nuclear attraction increases.<\/p>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">4<\/div><h2>Exception from Group 2 to Group 3<\/h2><\/div>\n        <p>A small drop in first ionisation energy occurs from <strong>Group 2 to Group 3<\/strong>. In Period 3, this is the drop from magnesium to aluminium.<\/p>\n        <p>Magnesium loses an electron from the <strong>3s<\/strong> subshell, whereas aluminium loses an electron from the <strong>3p<\/strong> subshell. The 3p electron is higher in energy and is slightly shielded by the 3s electrons, so it is easier to remove.<\/p>\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\/05\/Understanding-electron-arrangements-in-group-2-and-group-3.webp\" alt=\"Electron arrangement comparison for Group 2 and Group 3 ionisation energy exception\">\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <p>The first electron removed from aluminium is in a higher-energy 3p subshell rather than the 3s subshell.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-definition-box\">\n          <p><strong>Do not explain this dip by saying aluminium has more shielding overall.<\/strong> The key point is that the removed electron is in a higher-energy 3p subshell and is slightly shielded by the 3s electrons.<\/p>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">5<\/div><h2>Exception from Group 5 to Group 6<\/h2><\/div>\n        <p>A second small drop occurs from <strong>Group 5 to Group 6<\/strong>. In Period 3, this is the drop from phosphorus to sulfur.<\/p>\n        <p>Phosphorus has three 3p electrons, each in a separate 3p orbital. Sulfur has four 3p electrons, so one 3p orbital contains a pair of electrons. Repulsion between the paired electrons makes one of them easier to remove.<\/p>\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\/05\/5-and-6-electron-configuration-comparison.webp\" alt=\"Electron configuration comparison for the Group 5 to Group 6 ionisation energy exception\">\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <p>The paired 3p electrons in sulfur repel each other, lowering the energy needed to remove one electron.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Exam focus:<\/strong> this exception is caused by electron-electron repulsion within a paired 3p orbital, not by a change in shell.<\/p>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">6<\/div><h2>Second Ionisation Energy Across Period 3<\/h2><\/div>\n        <p>The pattern in <strong>second ionisation energy<\/strong> is similar to the pattern in first ionisation energy, but it appears shifted one place to the left.<\/p>\n        <p>This is because the second ionisation energy removes an electron from a <strong>1+ ion<\/strong>. For example, sodium has a very high second ionisation energy because its second electron is removed from an inner shell closer to the nucleus.<\/p>\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\/05\/Second-ionisation-energy-across-Period-3.webp\" alt=\"Second ionisation energy graph across Period 3\">\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <p>Group 1 elements appear at high points for second ionisation energy because the second electron is removed from an inner shell.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Trend<\/th>\n                <th>Reason<\/th>\n                <th>Exam phrase to use<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Trend\">First ionisation energy generally increases across a period<\/td>\n                <td data-label=\"Reason\">Nuclear charge increases while shielding remains similar.<\/td>\n                <td data-label=\"Exam phrase to use\">Stronger attraction between the nucleus and the outer electron.<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Trend\">Small drop from Group 2 to Group 3<\/td>\n                <td data-label=\"Reason\">The electron removed is from a higher-energy p subshell.<\/td>\n                <td data-label=\"Exam phrase to use\">The 3p electron is higher in energy and slightly shielded by 3s electrons.<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Trend\">Small drop from Group 5 to Group 6<\/td>\n                <td data-label=\"Reason\">A paired p electron experiences extra repulsion.<\/td>\n                <td data-label=\"Exam phrase to use\">Repulsion between paired electrons makes one electron easier to remove.<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Trend\">Second ionisation energy graph is shifted<\/td>\n                <td data-label=\"Reason\">Electrons are removed from 1+ ions rather than neutral atoms.<\/td>\n                <td data-label=\"Exam phrase to use\">The next electron may be removed from an inner shell closer to the nucleus.<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">7<\/div><h2>Common Exam Points<\/h2><\/div>\n        <ul>\n          <li>Across a period, <strong>proton number increases<\/strong>, so nuclear charge increases.<\/li>\n          <li>Electrons are added to the <strong>same main energy level<\/strong>, so shielding remains roughly constant.<\/li>\n          <li>Atomic radius decreases because the outer electrons are pulled closer to the nucleus.<\/li>\n          <li>First ionisation energy generally increases because the outer electron is more strongly attracted.<\/li>\n          <li>The Group 2 to Group 3 drop is caused by removal from a higher-energy p subshell.<\/li>\n          <li>The Group 5 to Group 6 drop is caused by repulsion between paired p electrons.<\/li>\n          <li>For second ionisation energy, remember that electrons are removed from positive ions, not neutral atoms.<\/li>\n        <\/ul>\n      <\/article>\n\n      <section class=\"ols-h5p-card\">\n        <h2>Check Your Understanding<\/h2>\n        <p>Use these short tasks to test the key trends and exceptions before moving on to ionisation energy trends down a group.<\/p>\n\n        <div class=\"ols-h5p-grid\">\n          <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-271\" class=\"h5p-iframe\" data-content-id=\"271\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"T2.4.9 Drag: Trends in a Period Key Concepts\"><\/iframe><\/div><\/div>\n          <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-272\" class=\"h5p-iframe\" data-content-id=\"272\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"T2.4.10 MCQ: First Ionisation Energy Across a Period\"><\/iframe><\/div><\/div>\n          <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-273\" class=\"h5p-iframe\" data-content-id=\"273\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"T2.4.11 MCQ: Small Drop from Group 2 to Group 3\"><\/iframe><\/div><\/div>\n          <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-274\" class=\"h5p-iframe\" data-content-id=\"274\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"T2.4.12 MCQ: Small Drop from Group 5 to Group 6\"><\/iframe><\/div><\/div>\n          <div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-350\" class=\"h5p-iframe\" data-content-id=\"350\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ionisation Energy: Trends in a Period Dialog Cards\"><\/iframe><\/div><\/div>\n        <\/div>\n      <\/section>\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>Atomic radius decreases<\/strong> across a period because nuclear charge increases while shielding remains similar.<\/li>\n          <li><strong>First ionisation energy generally increases<\/strong> because the outer electron is more strongly attracted to the nucleus.<\/li>\n          <li><strong>Group 2 to Group 3 drop:<\/strong> the electron is removed from a higher-energy p subshell.<\/li>\n          <li><strong>Group 5 to Group 6 drop:<\/strong> a paired p electron experiences extra repulsion and is easier to remove.<\/li>\n          <li><strong>Second ionisation energy<\/strong> follows a related pattern, but the graph is shifted because electrons are removed from 1+ ions.<\/li>\n          <li><strong>Best exam answers<\/strong> use the terms nuclear charge, shielding, distance from the nucleus and electron-electron repulsion accurately.<\/li>\n        <\/ul>\n      <\/section>\n\n      <section class=\"ols-course-cta-atomic-structure\" id=\"olsCourseCtaAtomicStructure002\">\n  <style>\n    .ols-course-cta-atomic-structure,\n    .ols-course-cta-atomic-structure * {\n      box-sizing: border-box;\n    }\n\n    .ols-course-cta-atomic-structure {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --body-text: #1f2937;\n      --grey-text: #667085;\n      --border: rgba(28, 36, 75, 0.14);\n   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<h3>Progress tracking<\/h3>\n        <p>\n          Identify strengths and weaknesses across atomic structure, isotopes, mass spectrometry, ionisation energies, electron configuration and periodic trends with targeted reporting.\n        <\/p>\n      <\/div>\n\n    <\/div>\n\n    <div class=\"ols-course-animation-wrap\">\n\n      <h3 class=\"ols-course-animation-title\">\n        See how the course works\n      <\/h3>\n\n      <div class=\"ols-animation-frame-shell\">\n        <iframe id=\"olsCourseAnimationFrame002\" title=\"OLS course preview animation\" loading=\"lazy\" referrerpolicy=\"no-referrer\">\n        <\/iframe>\n\n        <button class=\"ols-animation-start-overlay\" id=\"olsCourseAnimationStart002\" type=\"button\" aria-label=\"Play OLS course preview animation\">\n          <span class=\"ols-animation-start-content\">\n            <span class=\"ols-animation-play-circle\" aria-hidden=\"true\">\n              <span class=\"ols-animation-play-icon\"><\/span>\n            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src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Virtual-Lesson.mp4\" muted=\"\" playsinline=\"\" preload=\"auto\">\n            <\/video>\n          <\/div>\n        <\/div>\n      <\/section>\n    <\/div>\n  <\/section>\n\n  <section id=\"sceneMcq\" class=\"ols-scene\">\n    <div class=\"ols-mcq-stage\">\n      <div id=\"mcqIntroScreen\" class=\"ols-mcq-intro-screen\">\n        <div class=\"ols-mcq-intro-title-wrap\">\n          <h1 class=\"ols-mcq-intro-title\">\n            <span id=\"mcqIntroTitleText\"><\/span><span id=\"mcqIntroTitleCursor\" class=\"ols-title-cursor\"><\/span>\n          <\/h1>\n        <\/div>\n      <\/div>\n\n      <div class=\"ols-mcq-scale-wrap\">\n        <main id=\"mcqScreen\" class=\"ols-mcq-screen\">\n          <section class=\"mcq-question-area\">\n            <p class=\"mcq-question-lead\">Some successive ionisation energies for element X are shown.<\/p>\n\n            <table class=\"mcq-data-table\">\n              <thead>\n                <tr>\n                  <th>Ionisation energy<\/th>\n                  <th>Value \/ kJ mol<sup>\u22121<\/sup><\/th>\n                <\/tr>\n              <\/thead>\n              <tbody>\n                <tr><td>1st<\/td><td>738<\/td><\/tr>\n                <tr><td>2nd<\/td><td>1451<\/td><\/tr>\n                <tr><td>3rd<\/td><td>7733<\/td><\/tr>\n                <tr><td>4th<\/td><td>10543<\/td><\/tr>\n              <\/tbody>\n            <\/table>\n\n            <p class=\"mcq-question-main\">Which formula is most likely for the fluoride of element X?<\/p>\n\n            <div class=\"mcq-options-wrap\">\n              <div id=\"mcqWrongOption\" class=\"mcq-option-row\">\n                <span class=\"mcq-radio\"><\/span>\n                <span class=\"mcq-radio-ripple\"><\/span>\n                <span class=\"mcq-option-label\">XF<\/span>\n\n                <span id=\"mcqSpecificFeedback\" class=\"mcq-specific-feedback\">\n                  <span id=\"mcqCrossIcon\" class=\"mcq-cross-icon\">\u00d7<\/span>\n                  <span class=\"mcq-specific-feedback-text\">\n                    <span id=\"mcqSpecificText\"><\/span><span id=\"mcqSpecificCursor\" class=\"cursor mcq-specific-cursor\" style=\"display:none;\"><\/span>\n                  <\/span>\n                <\/span>\n              <\/div>\n\n              <div class=\"mcq-option-row\"><span class=\"mcq-radio\"><\/span><span class=\"mcq-radio-ripple\"><\/span><span class=\"mcq-option-label\">XF<sub>2<\/sub><\/span><\/div>\n              <div class=\"mcq-option-row\"><span class=\"mcq-radio\"><\/span><span class=\"mcq-radio-ripple\"><\/span><span class=\"mcq-option-label\">XF<sub>3<\/sub><\/span><\/div>\n              <div class=\"mcq-option-row\"><span class=\"mcq-radio\"><\/span><span class=\"mcq-radio-ripple\"><\/span><span class=\"mcq-option-label\">XF<sub>4<\/sub><\/span><\/div>\n            <\/div>\n\n            <button id=\"mcqSubmitButton\" class=\"mcq-submit-button\">Submit answer<\/button>\n          <\/section>\n\n          <section id=\"mcqGeneralFeedback\" class=\"mcq-general-feedback\">\n            <span id=\"mcqGeneralText\"><\/span><span id=\"mcqGeneralCursor\" class=\"cursor mcq-general-cursor\" style=\"display:none;\"><\/span>\n          <\/section>\n        <\/main>\n      <\/div>\n    <\/div>\n  <\/section>\n\n  <section id=\"sceneSaq\" class=\"ols-scene\">\n    <div class=\"ols-saq-stage\">\n      <div id=\"saqIntro\" class=\"ols-saq-intro\">\n        <div class=\"ols-saq-intro-title-wrap\">\n          <h1 class=\"ols-saq-intro-title\">\n            <span id=\"saqIntroTitleText\"><\/span><span id=\"saqIntroTitleCursor\" class=\"ols-title-cursor\"><\/span>\n          <\/h1>\n        <\/div>\n      <\/div>\n\n      <div class=\"ols-saq-scale-wrap\">\n        <main id=\"saqScreen\" class=\"ols-saq-screen\">\n          <section class=\"saq-question-area\">\n            <div class=\"saq-question-text\">\n              <strong>(ii)<\/strong>&nbsp;&nbsp; First ionisation energy generally increases across Period 3.<br>\n              Explain this trend in terms of nuclear charge, shielding and atomic radius.\n            <\/div>\n\n            <div class=\"saq-student-box\">\n              <span id=\"saqStudentText\"><\/span><span id=\"saqStudentCursor\" class=\"cursor\"><\/span>\n            <\/div>\n          <\/section>\n\n          <section id=\"saqTeacherFeedback\" class=\"saq-teacher-feedback\">\n            <span id=\"saqTeacherText\"><\/span><span id=\"saqTeacherCursor\" class=\"cursor saq-teacher-cursor\" style=\"display:none;\"><\/span>\n          <\/section>\n        <\/main>\n      <\/div>\n    <\/div>\n  <\/section>\n\n<\/div>\n\n<script>\n(function(){\n  const nativeSetTimeout=window.setTimeout.bind(window);\n  const nativeClearTimeout=window.clearTimeout.bind(window);\n  let nextTimerId=1;\n  let paused=false;\n  const timers=new Map();\n\n  function runTimer(timerId){\n    const 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Electrons are added to the same main energy level, so shielding remains similar. The attraction between the nucleus and the outer electron becomes stronger.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>Why does atomic radius decrease across a period?<\/h3>\n            <p>The increasing nuclear charge pulls the electron cloud closer to the nucleus. Since the added electrons enter the same main energy level, there is no major extra inner-shell shielding.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>Why is aluminium lower than magnesium in first ionisation energy?<\/h3>\n            <p>Aluminium loses a 3p electron, while magnesium loses a 3s electron. The 3p electron is higher in energy and is slightly shielded by the 3s electrons, so it is easier to remove.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>Why is sulfur lower than phosphorus in first ionisation energy?<\/h3>\n            <p>Sulfur has a pair of electrons in one 3p orbital. Repulsion between the paired electrons makes one electron easier to remove, so the first ionisation energy is slightly lower.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>Why is sodium's second ionisation energy very high?<\/h3>\n            <p>After sodium loses its first electron, the next electron would be removed from an inner shell closer to the nucleus. It is less shielded and more strongly attracted, so much more energy is needed.<\/p>\n          <\/div>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-related-card\">\n        <h2>Related Topics<\/h2>\n        <p>Continue through Topic 2 by linking this page to the rest of ionisation energy and atomic structure.<\/p>\n        <div class=\"ols-related-grid\">\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/introduction-to-ionisation-energy\/\">Introduction to Ionisation Energy<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/successive-ionisation-energy\/\">Successive Ionisation Energy<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/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-international\/topic-2-atomic-structure-and-the-periodic-table\/quantum-model\/\">Quantum Model<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/mass-spectrometry\/\">Mass Spectrometry<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Bonding &amp; Structure<\/a>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-attribution-card\">\n        <p><strong>Copyright notice:<\/strong> This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.<\/p>\n      <\/section>\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-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/trends-in-a-period\/#webpage\",\n                        \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/trends-in-a-period\/\",\n                        \"name\": \"Trends in a Period | Edexcel International A Level Chemistry Revision Notes\",\n                        \"description\": \"Revision notes explaining first ionisation energy trends 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                           \"text\": \"First ionisation energy generally increases across a period because nuclear charge increases as proton number increases. Electrons are added to the same main energy level, so shielding remains similar and the attraction between the nucleus and the outer electron increases.\"\n                                        }\n                                },\n                                {\n                                        \"@type\": \"Question\",\n                                        \"name\": \"Why does atomic radius decrease across a period?\",\n                                        \"acceptedAnswer\": {\n                                                \"@type\": \"Answer\",\n                                                \"text\": \"Atomic radius decreases across a period because the number of protons increases while added electrons enter the same main energy level. Shielding does not increase significantly, so stronger nuclear attraction pulls the electron cloud closer to the nucleus.\"\n                                        }\n                                },\n                                {\n                                        \"@type\": \"Question\",\n                                        \"name\": \"Why is there a small drop in first ionisation energy from magnesium to aluminium?\",\n                                        \"acceptedAnswer\": {\n                                                \"@type\": \"Answer\",\n                                                \"text\": \"Aluminium's outer electron is removed from a 3p subshell, whereas magnesium's outer electron is removed from a 3s subshell. The 3p electron is higher in energy and slightly shielded by the 3s electrons, so it is easier to remove.\"\n                                        }\n                                },\n                                {\n                                        \"@type\": \"Question\",\n                                        \"name\": \"Why is there a small drop in first ionisation energy from phosphorus to sulfur?\",\n                                        \"acceptedAnswer\": {\n                                                \"@type\": \"Answer\",\n                                                \"text\": \"Sulfur has a pair of electrons in one 3p orbital. Repulsion between the paired electrons makes one of them easier to remove, so sulfur has a slightly lower first ionisation energy than phosphorus.\"\n                                        }\n                                },\n                                {\n                                        \"@type\": \"Question\",\n                                        \"name\": \"Why do second ionisation energy patterns shift one place to the left?\",\n                                        \"acceptedAnswer\": {\n                                                \"@type\": \"Answer\",\n                                                \"text\": \"Second ionisation energy removes an electron from a 1+ ion rather than from a neutral atom. This means each element's second ionisation energy pattern resembles the first ionisation energy pattern of the element one place to the right, so the graph appears shifted one place to the left.\"\n                                        }\n                                }\n                        ]\n                },\n                {\n                        \"@type\": \"ItemList\",\n                        \"@id\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/ionisation-energy\/trends-in-a-period\/#relatedtopics\",\n                        \"name\": \"Related Ionisation Energy Revision Notes\",\n                        \"itemListElement\": [\n                                {\n                                        \"@type\": \"ListItem\",\n                                        \"position\": 1,\n                                        \"item\": {\n                      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