{"id":12477,"date":"2026-10-03T08:32:23","date_gmt":"2026-10-03T07:32:23","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-trends-and-covalent-character\/"},"modified":"2026-10-04T23:04:43","modified_gmt":"2026-10-04T22:04:43","slug":"lattice-energy-trends-and-covalent-character","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-trends-and-covalent-character\/","title":{"rendered":"Lattice Energy Trends and Covalent Character"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-kinetics-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      --gold: 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20px;\n      box-shadow: var(--inner-shadow);\n    }\n.ols-faq-item h3 {\n      margin: 0 0 8px;\n      font-size: 20px;\n      line-height: 1.3;\n      color: var(--navy);\n    }\n.ols-faq-item p {\n      margin: 0;\n      font-size: 16px;\n      line-height: 1.65;\n      color: var(--body-text);\n    }\n.ols-faq-card,\n      .ols-quicksnap-card,\n      .ols-attribution-card {\n        padding: 24px 18px;\n        border-radius: 22px;\n      }\n  <\/style>\n\n    <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Topic 12 Entropy and Energetics<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/\">Topic 12 Entropy and Energetics Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/entropy-and-the-direction-of-change\/\">Entropy and the Direction of Change<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/calculating-entropy-changes\/\">Calculating Entropy Changes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/feasibility-gibbs-energy-and-temperature\/\">Feasibility and Temperature<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/thermodynamic-and-kinetic-stability\/\">Thermodynamic and Kinetic Stability<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-and-born-haber-cycles\/\">Lattice Energy and Born\u2013Haber Cycles<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-trends-and-covalent-character\/\">Lattice Energy Trends and Covalent Character<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/enthalpy-of-solution-and-hydration\/\">Enthalpy of Solution and Hydration<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/predicting-solubility\/\">Predicting Solubility<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Other Sections<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/\">Topic 6 Energetics (AS)<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-11-kinetics\/\">Topic 11 Kinetics<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/\">Topic 8B Groups 1 and 2<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/\">Core Practicals<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n\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\"], 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<h1>Lattice Energy Trends and Covalent Character<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to what decides the size of lattice energy: ionic charge and ionic radius, the perfect ionic model, why experimental Born\u2013Haber values differ from theoretical ones, polarisation of anions by cations and the covalent character it produces.<\/p>\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Exam board: Edexcel International<\/div>\n<div class=\"ols-badge\">Unit 4: WCH14\/01<\/div>\n<div class=\"ols-badge\">Topic 12: Entropy and Energetics<\/div>\n<\/div>\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 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\">\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        View LinkedIn Profile\n      <\/a>\n\n    <\/div>\n\n  <\/div>\n      <\/header>\n\n      <article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Charge and Radius<\/h2>\n<\/div>\n<p>The lattice energy comes from the electrostatic attraction between oppositely charged ions. Two things decide how exothermic it is: the charges on the ions and their radii.<\/p>\n<ul>\n<li>The force between two charges grows with the <strong>size of the charges<\/strong>, so more highly charged ions attract more strongly.<\/li>\n<li>The force falls as the <strong>distance between them<\/strong> grows. Smaller ions sit closer together, so the attraction is stronger.<\/li>\n<\/ul>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Compound<\/th><th>Ions<\/th><th>Sum of ionic radii \/ pm<\/th><th>Lattice energy \/ kJ mol\u207b\u00b9<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>LiF<\/strong><\/td><td>Li\u207a, F\u207b<\/td><td>209<\/td><td>\u22121037<\/td><\/tr>\n<tr><td><strong>NaF<\/strong><\/td><td>Na\u207a, F\u207b<\/td><td>235<\/td><td>\u2212918<\/td><\/tr>\n<tr><td><strong>NaCl<\/strong><\/td><td>Na\u207a, Cl\u207b<\/td><td>283<\/td><td>\u2212787<\/td><\/tr>\n<tr><td><strong>NaBr<\/strong><\/td><td>Na\u207a, Br\u207b<\/td><td>298<\/td><td>\u2212742<\/td><\/tr>\n<tr><td><strong>KBr<\/strong><\/td><td>K\u207a, Br\u207b<\/td><td>334<\/td><td>\u2212671<\/td><\/tr>\n<tr><td><strong>MgO<\/strong><\/td><td>Mg\u00b2\u207a, O\u00b2\u207b<\/td><td>212<\/td><td>\u22123791<\/td><\/tr>\n<tr><td><strong>CaO<\/strong><\/td><td>Ca\u00b2\u207a, O\u00b2\u207b<\/td><td>240<\/td><td>\u22123401<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Read the table two ways.<\/p>\n<ul>\n<li><strong>Radius:<\/strong> down the first five rows the charges are all 1+ and 1\u2212, and the lattice energy becomes less exothermic as the ions get bigger.<\/li>\n<li>LiF is the most exothermic and KBr the least, because the ion centres are further apart.<\/li>\n<li><strong>Charge:<\/strong> compare NaCl with MgO, whose ions are a similar size. Doubling both charges makes the lattice energy nearly five times more exothermic.<\/li>\n<li>Charge matters more than radius.<\/li>\n<\/ul>\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\/09\/entropy-t12-15-lefactors.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-15-lefactors.jpg\" alt=\"Poster of the two factors that make a lattice energy more exothermic: three 1+\/1\u2212 ion pairs drawn to scale with the distance\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-15-lefactors.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The two factors that decide the size of a lattice energy: smaller ions sit closer together, and more highly charged ions attract far more strongly, with values for five compounds.<\/p><\/div>\n<\/div>\n<p>The same two factors set the melting point, so MgO (2852 \u00b0C) is used as a refractory lining while NaCl melts at 801 \u00b0C.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> More exothermic lattice energy: higher ionic charge, smaller ionic radius (higher charge density on both ions). State both the factor and the reason: &#8220;smaller ions, so the ions are closer and the electrostatic attraction is stronger&#8221;.<\/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>Check: Comparing Values From Charge and Radius<\/h2>\n<p>Rank and explain the lattice energy of pairs of compounds not on this page from the charges and radii of their ions.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1114\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>The Perfect Ionic Model<\/h2>\n<\/div>\n<p>A lattice energy can be worked out in two independent ways.<\/p>\n<ul>\n<li>The <strong>experimental value<\/strong> comes from the Born\u2013Haber cycle. Every step in it is measured, so the result is what the real lattice releases.<\/li>\n<li>The <strong>theoretical value<\/strong> comes from electrostatics. It assumes the <strong>perfect ionic model<\/strong>.<\/li>\n<\/ul>\n<p>The perfect ionic model assumes that:<\/p>\n<ul>\n<li>the ions are perfect spheres<\/li>\n<li>each charge is concentrated at the centre of its ion (a point charge)<\/li>\n<li>the charge is spread evenly over the ion<\/li>\n<li>the only attraction is electrostatic.<\/li>\n<\/ul>\n<p>Knowing the charges, the radii and the geometry of the lattice, the energy released can then be calculated.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Compound<\/th><th>Experimental (Born\u2013Haber) \/ kJ mol\u207b\u00b9<\/th><th>Theoretical (perfect ionic model) \/ kJ mol\u207b\u00b9<\/th><th>Gap \/ kJ mol\u207b\u00b9<\/th><th>What it shows<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Sodium chloride<\/strong><\/td><td>\u2212787<\/td><td>\u2212766<\/td><td>21<\/td><td>the two values agree closely: close to purely ionic<\/td><\/tr>\n<tr><td><strong>Silver chloride<\/strong><\/td><td>\u2212905<\/td><td>\u2212770<\/td><td>135<\/td><td>experimental markedly more exothermic: some covalent character<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>For many compounds the two values agree closely, and the compound is close to <strong>purely ionic<\/strong>. For others the experimental value is markedly <strong>more exothermic<\/strong> than the theoretical one.<\/p>\n<p>The real silver chloride lattice is held together more strongly than pure electrostatics predicts, so there must be extra bonding. The electrons are partly shared between the ions and the bonding has some <strong>covalent character<\/strong>.<\/p>\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\/09\/entropy-t12-08-covalency.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-08-covalency.jpg\" alt=\"Table comparing experimental (Born\u2013Haber) and theoretical (perfect ionic model) lattice energies for sodium and silver halides\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-08-covalency.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Experimental (Born\u2013Haber) against theoretical (perfect ionic model) values for the sodium and silver halides, beside a small cation polarising a large anion.<\/p><\/div>\n<\/div>\n<p>The size of the gap is a measure of the degree of covalent bonding.<\/p>\n<ul>\n<li>A difference of a few per cent means an almost purely ionic compound.<\/li>\n<li>A difference of 15 to 20 per cent, as in the silver halides, means substantial covalent character.<\/li>\n<li>Note the direction: the experimental value is always the <strong>more exothermic<\/strong> of the two, because sharing electrons adds to the attraction and never subtracts from it.<\/li>\n<\/ul>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> Theoretical value: &#8220;assumes the perfect ionic model: spherical ions with the charge evenly distributed, attracted only electrostatically&#8221;. Comparison: &#8220;the experimental value is more exothermic than the theoretical value, so there is additional covalent bonding&#8221;.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>Polarisation<\/h2>\n<\/div>\n<p>The extra bonding comes from <strong>polarisation<\/strong>.<\/p>\n<ol>\n<li>A cation attracts the electron cloud of the anion next to it.<\/li>\n<li>If the pull is strong enough, the cloud is distorted towards the cation.<\/li>\n<li>Electron density builds up between the two nuclei, which is the beginning of a covalent bond.<\/li>\n<\/ol>\n<p>The anion is said to be polarised and the cation is polarising.<\/p>\n<p>The <strong>polarising power<\/strong> of a cation depends on its <strong>charge density<\/strong>.<\/p>\n<ul>\n<li>A small, highly charged cation (Li\u207a, Be\u00b2\u207a, Mg\u00b2\u207a, Al\u00b3\u207a, Ag\u207a) has a strong electric field close to its surface and distorts anions strongly.<\/li>\n<li>A large 1+ cation (K\u207a, Cs\u207a) hardly distorts them at all.<\/li>\n<\/ul>\n<p>The <strong>polarisability<\/strong> of an anion depends on its size and charge.<\/p>\n<ul>\n<li>A large anion (I\u207b, S\u00b2\u207b) holds its outer electrons loosely and far from its nucleus, so it is easily distorted.<\/li>\n<li>A highly charged anion is distorted more easily than a singly charged one of the same size.<\/li>\n<\/ul>\n<p>The greater the polarisation, the greater the covalent character and the larger the gap between the theoretical and experimental values.<\/p>\n<!-- 3D card: anion-polarisation (3 Oct 2026, reused) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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right:14px;}\n}\n\n.ols-cc-pol-001{\n  font-family:'Poppins',system-ui,-apple-system,'Segoe UI',Roboto,Helvetica,Arial,sans-serif;\n  margin:10px auto 0; text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-pol-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-pol-001 a:hover{text-decoration:underline;}\n<\/style>\n\n<div class=\"pol-head\">\n  <h2 class=\"pol-title\">Polarisation of Anions: AlCl<sub>3<\/sub> and AlF<sub>3<\/sub><\/h2>\n  <p class=\"pol-sub\">See how the small, highly charged Al<sup>3+<\/sup> ion pulls on the electron clouds of the anions around it, distorting large Cl<sup>&minus;<\/sup> far more than small F<sup>&minus;<\/sup>.<\/p>\n<\/div>\n\n<div class=\"pol-stage\" id=\"polStage\">\n  <canvas class=\"pol-canvas\" id=\"polCanvas\"><\/canvas>\n  <div class=\"pol-overlay\" id=\"polOverlay\"><\/div>\n  <div class=\"pol-caption\" id=\"polCaption\"><\/div>\n  <div class=\"pol-hint\" id=\"polHint\">Drag to rotate<\/div>\n<\/div>\n\n<div class=\"pol-controls\">\n  <div class=\"pol-row\">\n    <span class=\"pol-rowlab\">Compound<\/span>\n    <div class=\"pol-seg\" role=\"group\" aria-label=\"Compound\">\n      <button type=\"button\" data-c=\"AlCl3\" aria-pressed=\"true\">Aluminium chloride, AlCl<sub>3<\/sub><\/button>\n      <button type=\"button\" data-c=\"AlF3\" aria-pressed=\"false\">Aluminium fluoride, AlF<sub>3<\/sub><\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"pol-row\">\n    <span class=\"pol-rowlab\">Step<\/span>\n    <div class=\"pol-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\">Ions<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\">Polarise<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\">Compare<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"pol-row\">\n    <span class=\"pol-rowlab\">View<\/span>\n    <button type=\"button\" class=\"pol-pill\" id=\"polResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"pol-info\">\n  <h3 id=\"polInfoTitle\"><\/h3>\n  <p id=\"polInfoText\"><\/p>\n  <div class=\"pol-facts\" id=\"polFacts\"><\/div>\n  <div class=\"pol-key\">\n    <span><i style=\"background:#787e8c\"><\/i>Al<sup>3+<\/sup><\/span>\n    <span><i style=\"background:#3aa048\"><\/i>Cl<sup>&minus;<\/sup><\/span>\n    <span><i style=\"background:#96cd7a\"><\/i>F<sup>&minus;<\/sup><\/span>\n    <span><i style=\"background:rgba(40,128,56,0.35); border:1.5px dotted rgba(40,128,56,0.9)\"><\/i>Electron density<\/span>\n    <span><i style=\"background:#ffffff; border:1.5px dashed rgba(28,36,75,0.6)\"><\/i>Undistorted cloud<\/span>\n    <span><i style=\"background:#1C244B\"><\/i>Ionic radius ruler<\/span>\n  <\/div>\n<\/div>\n<\/section>\n\n<p class=\"ols-cc-pol-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/anion-polarisation.js?v=20261003e\"><\/script>\n<p>The silver halides show both factors.<\/p>\n<ul>\n<li>Ag\u207a is more polarising than Na\u207a of similar size because its d electrons shield the nuclear charge poorly, so AgCl has far more covalent character than NaCl.<\/li>\n<li>Moving from AgCl to AgI the anion gets larger and more polarisable, so the gap grows further (140 kJ mol\u207b\u00b9 for AgI).<\/li>\n<\/ul>\n<p>The same reasoning explains why aluminium chloride and beryllium chloride behave as covalent compounds. A 3+ or a very small 2+ cation polarises a chloride ion so strongly that the ionic model no longer describes the bonding.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Polarising cation: small radius, high charge. Polarisable anion: large radius, high charge. Polarisation \u2192 electron density shared between the nuclei \u2192 covalent character \u2192 experimental value more exothermic than the theoretical value.<\/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>Check: Polarisation and Covalent Character<\/h2>\n<p>Decide which cation is more polarising, which anion is more polarisable, and which of two compounds shows more covalent character, for ion pairs not used on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1115\" class=\"h5p-iframe\" data-content-id=\"1115\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Entropy and Energetics Summary: Polarisation and Covalent Character\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Say<\/h3>\n<ul>\n<li>&#8220;The lattice energy is more exothermic because the ions are smaller (or more highly charged), so the electrostatic attraction between them is stronger.&#8221;<\/li>\n<li>&#8220;The experimental value is more exothermic than the theoretical value, so the bonding has some covalent character.&#8221;<\/li>\n<li>&#8220;The cation polarises the anion: it distorts the electron cloud so that electron density lies between the nuclei.&#8221;<\/li>\n<\/ul>\n<h3>Do not say<\/h3>\n<ul>\n<li>&#8220;The lattice energy is bigger&#8221; without saying more exothermic or more negative.<\/li>\n<li>&#8220;The theoretical value is wrong&#8221; (it is right for a purely ionic compound; the compound is not purely ionic).<\/li>\n<li>&#8220;The anion polarises the cation.&#8221;<\/li>\n<\/ul>\n<h3>Watch for<\/h3>\n<ul>\n<li>A comparison where the two compounds differ in both charge and radius: deal with charge first, it dominates.<\/li>\n<li>A question that gives three values and asks which compound is most covalent: look for the largest percentage gap, not the most exothermic value.<\/li>\n<li>A cation such as Ag\u207a that polarises more than its radius suggests.<\/li>\n<\/ul>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Theoretical Against Experimental<\/h2>\n<p>Given theoretical and Born\u2013Haber values for compounds not on this page, say which is more covalent and explain the difference in terms of polarisation.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1116\" class=\"h5p-iframe\" data-content-id=\"1116\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Entropy and Energetics Explain: The Gap Between Theoretical and Experimental Values\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check what decides the size of lattice energy and how covalent character shows up.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why is MgO&#8217;s lattice energy so much bigger than NaCl&#8217;s?<\/h3>\n<p>Both ions in MgO carry a double charge, so the electrostatic attraction between them is roughly four times that between singly charged ions. On top of that Mg\u00b2\u207a and O\u00b2\u207b are smaller than Na\u207a and Cl\u207b, so the ions sit closer together. Higher charge and smaller radius together give a value roughly five times larger.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why does the Born\u2013Haber value differ from the theoretical one?<\/h3>\n<p>The theoretical value assumes a perfect ionic model: spherical ions with the charge spread evenly, held together only by electrostatic attraction. In a real lattice the cation pulls the electron cloud of the anion towards itself, adding some covalent bonding and making the lattice more stable. The experimental (Born\u2013Haber) value is therefore more exothermic, and the gap measures the covalent character.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What makes a cation good at polarising an anion?<\/h3>\n<p>A high charge density: a small cation with a large charge, such as Al\u00b3\u207a or Be\u00b2\u207a. The anion is polarised most easily when it is large and highly charged, such as I\u207b or S\u00b2\u207b, because its outer electrons are far from the nucleus and loosely held. Silver iodide shows far more covalent character than sodium fluoride for both reasons.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why does lattice energy fall from LiF to CsI?<\/h3>\n<p>Because the ions get bigger down both groups, so the distance between the centres of the cation and the anion increases and the electrostatic attraction weakens. The charges stay the same, so radius is the only thing changing. The same argument explains why the value falls from NaF to NaI along a series with the same cation.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I use the difference between the two values in an answer?<\/h3>\n<p>Quote both numbers, say which one is the experimental Born\u2013Haber value, and give the difference as a percentage or in kJ mol\u207b\u00b9. A small difference means the compound is close to purely ionic; a large one, as for the silver halides, means significant covalent character caused by polarisation of the anion by the cation.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 12 Entropy and Energetics Pages<\/h2>\n<p>Use these pages to connect the ideas across Topic 12 Entropy and Energetics and the rest of the course.<\/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-12-entropy-and-energetics\/entropy-and-the-direction-of-change\/\">Entropy and the Direction of Change<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/calculating-entropy-changes\/\">Calculating Entropy Changes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/feasibility-gibbs-energy-and-temperature\/\">Feasibility and Temperature<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/thermodynamic-and-kinetic-stability\/\">Thermodynamic and Kinetic Stability<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-and-born-haber-cycles\/\">Lattice Energy and Born\u2013Haber Cycles<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/enthalpy-of-solution-and-hydration\/\">Enthalpy of Solution and Hydration<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/predicting-solubility\/\">Predicting Solubility<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/\">Topic 12 Entropy and Energetics Overview<\/a>\n<\/div>\n<\/section>\n<section class=\"ols-attribution-card\">\n        <p><strong>Copyright notice:<\/strong> This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. 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Energetics \/ Lattice Energy Trends and Covalent Character Lattice Energy Trends and Covalent Character A concise revision guide to what decides the size of lattice energy: ionic charge and ionic radius, the perfect ionic model, why experimental Born\u2013Haber values differ from 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