{"id":8170,"date":"2026-09-16T17:33:56","date_gmt":"2026-09-16T16:33:56","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/london-forces\/"},"modified":"2026-09-22T09:59:40","modified_gmt":"2026-09-22T08:59:40","slug":"london-forces","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/london-forces\/","title":{"rendered":"London Forces"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-nature-of-covalent-bonding-9ch0-page\">\n  <style>\n    .ols-revision-page {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --soft-blue: #eef4ff;\n      --soft-red: #fff7f7;\n      --soft-purple: #f7f0ff;\n      --soft-green: #f0f7f1;\n      --soft-orange: #fff7ed;\n      --grey-text: #667085;\n      --body-text: #1f2937;\n      --border: rgba(28, 36, 75, 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.ols-figure-placeholder .ols-figure-caption p { margin: 10px 0 0; font-size: 14px; color: #667085; text-align: center; }\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>3.6 Intermolecular Forces<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/\">Part overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/london-forces\/\">London Forces<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/permanent-dipole-dipole-forces\/\">Permanent Dipole-Dipole Forces<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/hydrogen-bonding\/\">Hydrogen Bonding<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/anomalous-properties-of-water\/\">Anomalous Properties of Water<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/boiling-temperature-trends\/\">Boiling Point Trends<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/choosing-solvents\/\">Choosing Solvents<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Other Topic 3 Parts<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/\">Topic 3 overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/electronegativity\/\">3.1 Electronegativity<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/ionic-bonding\/\">3.2 Ionic Bonding<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/metallic-bonding\/\">3.3 Metallic Bonding<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/covalent-bonding\/\">3.4 Covalent Bonding<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/shapes-of-molecules\/\">3.5 Shapes of Molecules<\/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 = 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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/\">Cambridge International (CIE)<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/\">Topic 3 Chemical Bonding<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/\">3.6 Intermolecular Forces<\/a> \/\n<span>London Forces<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>London Forces<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to instantaneous dipole\u2013induced dipole (id\u2013id) forces: how instantaneous dipoles induce dipoles in neighbouring molecules, why every molecule has them, and how the number of electrons and molecular shape control their strength, for Cambridge International A Level Chemistry.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">AS Level<\/div>\n<div class=\"ols-badge\">Topic 3: Chemical Bonding<\/div>\n<div class=\"ols-badge\">9701 Papers 1 and 2<\/div>\n<\/div>\n\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:<br><span>Dr. Mohammed Al-Fatah<\/span>\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      <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: Counting Electrons<\/h2>\n<p>Three quick GCSE questions on electrons before you meet the force that their movement creates.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"784\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>What London Forces Are<\/h2>\n<\/div>\n<p>Instantaneous dipole\u2013induced dipole (id\u2013id) forces are <strong>weak attractions between every molecule and every separate atom<\/strong>, caused by the constant movement of electrons. They are the only intermolecular force that exists between non-polar molecules such as N\u2082, CH\u2084 and I\u2082, and between the atoms of the noble gases.<\/p>\n<p>The electrons in a molecule are never still. At any instant the electron cloud can be slightly denser on one side than the other, so that side is momentarily \u03b4- and the opposite side \u03b4+. This is an <strong>instantaneous dipole<\/strong>. It lasts only for a fraction of a second, but while it exists it pushes the electrons of a neighbouring molecule away from the \u03b4- end and pulls them towards the \u03b4+ end, creating an <strong>induced dipole<\/strong> in the neighbour. The two dipoles then attract each other.<\/p>\n<p>Because the instantaneous dipole keeps changing, the induced dipole changes with it, so the attraction is always there on average even though no single dipole is permanent. This is why students should describe the force as <strong>instantaneous dipole-induced dipole<\/strong> rather than as a fixed charge on the molecule.<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/london-dispersion-forces-between-chlorine-molecules.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/london-dispersion-forces-between-chlorine-molecules.jpg\" alt=\"Two chlorine molecules: an instantaneous dipole in one induces a dipole in its neighbour, and the two attract\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>An instantaneous dipole in one molecule induces a dipole in its neighbour; the \u03b4- end of one attracts the \u03b4+ end of the other.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Definition:<\/strong> Id\u2013id forces are attractions between an instantaneous dipole in one molecule and the dipole it induces in a neighbouring molecule.<\/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: The Mechanism Step by Step<\/h2>\n<p>Build the explanation for two ethene molecules, in the order an examiner wants it.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-522\" class=\"h5p-iframe\" data-content-id=\"522\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"London Forces Drag: The Mechanism Between Two Ethene Molecules\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>Why Every Molecule Has Them<\/h2>\n<\/div>\n<p>Any particle with electrons can have an instantaneous dipole, so id\u2013id forces act between <strong>all<\/strong> molecules and atoms, polar or not. A polar molecule such as HCl has id\u2013id forces <em>as well as<\/em> the attraction between its permanent dipoles; the two forces add together.<\/p>\n<p>This is the point most often missed in exam answers. When a question asks for the intermolecular forces in a polar molecule, the answer must include id\u2013id forces alongside the stronger force, because they never switch off.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Common mistake:<\/strong> Writing that &#8220;polar molecules have permanent dipole forces instead of id\u2013id forces&#8221;. They have both. The word to use is <strong>in addition to<\/strong>.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Instead Of, or In Addition To<\/h2>\n<p>Judge one sentence written by a student about a polar molecule.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-525\" class=\"h5p-iframe\" data-content-id=\"525\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"London Forces True or False: A Polar Molecule Still Has Them\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card purple\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>What Decides Their Strength: Number of Electrons<\/h2>\n<\/div>\n<p>The more electrons a molecule has, the larger and more <strong>polarisable<\/strong> its electron cloud, so the larger the instantaneous and induced dipoles and the stronger the id\u2013id forces. Molecules with many electrons therefore need more energy to separate and have higher boiling points.<\/p>\n<p>The noble gases show the effect with no other force present. Each is a single atom, so the only attraction between the atoms is the id\u2013id forces between them, and the boiling point rises steadily as the number of electrons rises.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Noble gas<\/th><th>Electrons per atom<\/th><th>boiling point \/ \u00b0C<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Helium, He<\/strong><\/td><td>2<\/td><td>-269<\/td><\/tr>\n<tr><td><strong>Neon, Ne<\/strong><\/td><td>10<\/td><td>-246<\/td><\/tr>\n<tr><td><strong>Argon, Ar<\/strong><\/td><td>18<\/td><td>-186<\/td><\/tr>\n<tr><td><strong>Krypton, Kr<\/strong><\/td><td>36<\/td><td>-153<\/td><\/tr>\n<tr><td><strong>Xenon, Xe<\/strong><\/td><td>54<\/td><td>-108<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The halogens follow the same pattern. Fluorine and chlorine are gases at room temperature, bromine is a liquid and iodine is a solid, because the number of electrons per molecule rises from 18 in F\u2082 to 106 in I\u2082 and the id\u2013id forces between the molecules strengthen down the group.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Halogen<\/th><th>Electrons per molecule<\/th><th>melting point \/ \u00b0C<\/th><th>boiling point \/ \u00b0C<\/th><th>State at 25 \u00b0C<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>F\u2082<\/strong><\/td><td>18<\/td><td>-220<\/td><td>-188<\/td><td>gas<\/td><\/tr>\n<tr><td><strong>Cl\u2082<\/strong><\/td><td>34<\/td><td>-101<\/td><td>-34<\/td><td>gas<\/td><\/tr>\n<tr><td><strong>Br\u2082<\/strong><\/td><td>70<\/td><td>-7<\/td><td>59<\/td><td>liquid<\/td><\/tr>\n<tr><td><strong>I\u2082<\/strong><\/td><td>106<\/td><td>114<\/td><td>184<\/td><td>solid<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/boiling-temperature-and-electron-count-in-noble-gases-and-halogens.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/boiling-temperature-and-electron-count-in-noble-gases-and-halogens.jpg\" alt=\"Graph of boiling point against number of electrons for the noble gases and the halogens, both rising as the number of electrons increases\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Boiling point rises with the number of electrons for both series because the id\u2013id forces between the particles become stronger.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> I\u2082 has a higher boiling point than Cl\u2082 because I\u2082 molecules have more electrons, so the id\u2013id forces between the molecules are stronger and more energy is needed to overcome them.<\/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: Which of the Pair Boils Higher<\/h2>\n<p>Four quick comparisons using species that are not on this page, including one beyond the end of the noble gas table.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-523\" class=\"h5p-iframe\" data-content-id=\"523\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"London Forces Rapid Check: Which of the Pair Boils Higher\"><\/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>What Decides Their Strength: Shape and Surface Contact<\/h2>\n<\/div>\n<p>Molecules with the same number of electrons can still differ, because the attraction depends on how closely the electron clouds of neighbouring molecules can approach. <strong>Long, straight molecules can lie alongside each other with a large area of contact, so their id\u2013id forces are stronger than those between compact, branched molecules of the same formula.<\/strong><\/p>\n<p>Pentane and 2,2-dimethylpropane both have the formula C\u2085H\u2081\u2082 and 42 electrons. Pentane is a straight chain and boils at 36 \u00b0C; 2,2-dimethylpropane is almost spherical and boils at 10 \u00b0C. The branched molecule cannot get as close to its neighbours, so less energy is needed to separate them. The attraction between two molecules is really the sum of many small attractions between the parts of their surfaces that lie close together, so more contact simply means more of them to overcome. The <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/boiling-temperature-trends\/\">Boiling Temperature Trends<\/a> page uses this idea for the whole alkane series.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Two factors set the size of id\u2013id forces: the number of electrons (the larger factor) and the surface area of contact between neighbouring molecules.<\/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: Same Electrons, Different Shape<\/h2>\n<p>Two isomers with identical electron counts boil 19 \u00b0C apart. Decide why.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-524\" class=\"h5p-iframe\" data-content-id=\"524\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"London Forces MCQ: Branching in the Hexane Isomers\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>How Weak Is Weak?<\/h2>\n<\/div>\n<p>Id\u2013id forces are the weakest of the three intermolecular forces for molecules of a similar size, but &#8220;weak&#8221; is relative to covalent bonds, not to nothing. Roughly, a covalent bond needs 150 to 1000 kJ mol\u207b\u00b9 to break, a hydrogen bond 10 to 40 kJ mol\u207b\u00b9, and the id\u2013id forces between two small molecules only a few kJ mol\u207b\u00b9. For a very large molecule such as a polymer chain or I\u2082, however, the id\u2013id forces add up along the whole molecule and can exceed a hydrogen bond in total. Iodine shows it: nothing else holds one I\u2082 molecule to the next, yet about 62 kJ mol\u207b\u00b9 is needed to turn one mole of the solid straight into gas, more than any single hydrogen bond.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Interaction<\/th><th>Typical energy to break \/ kJ mol\u207b\u00b9<\/th><th>Acts between<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Covalent bond<\/strong><\/td><td>150 to 1000<\/td><td>atoms within a molecule<\/td><\/tr>\n<tr><td><strong>Ionic bond (lattice)<\/strong><\/td><td>600 to 4000<\/td><td>ions; this is the energy needed to separate one mole of the lattice into gaseous ions<\/td><\/tr>\n<tr><td><strong>Metallic bond<\/strong><\/td><td>80 to 900<\/td><td>metal ions and delocalised electrons<\/td><\/tr>\n<tr><td><strong>Hydrogen bond<\/strong><\/td><td>10 to 40<\/td><td>molecules with N-H, O-H or H-F<\/td><\/tr>\n<tr><td><strong>permanent dipole\u2013permanent dipole (pd\u2013pd) forces<\/strong><\/td><td>3 to 20<\/td><td>polar molecules<\/td><\/tr>\n<tr><td><strong>Id\u2013id forces<\/strong><\/td><td>1 to 20, rising with size<\/td><td>all molecules and atoms<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>This ladder explains why simple molecular substances melt and boil at low temperatures: only the intermolecular forces are overcome, never the covalent bonds inside the molecules. When ice melts or water boils, every O-H bond stays intact.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam trap:<\/strong> Never write that covalent bonds break when a molecular substance boils. The molecules separate; the bonds within them do not.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Names You Will See<\/h2>\n<\/div>\n<p>Cambridge uses van der Waals&#8217; forces as the family name for every intermolecular force other than a bond, and then divides the family into id\u2013id forces and pd\u2013pd forces, with hydrogen bonding as a special case of pd\u2013pd forces.<\/p>\n<p>Whatever name the question uses, the explanation is the same: moving electrons, an instantaneous dipole, an induced dipole in the neighbour, and an attraction whose size grows with the number of electrons and the area of contact.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> Use the name your specification uses: &#8220;id\u2013id forces&#8221;. Then say what causes them, because the name alone earns no marks.<\/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: The Ideas That Catch People Out<\/h2>\n<p>Pick the one accurate statement in each round to finish the page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-785\" class=\"h5p-iframe\" data-content-id=\"785\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"London Forces Summary: Names, Flickering Dipoles and Large Molecules\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">7<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Explain why boiling point rises down Group 17<\/h3><p>More electrons per molecule, larger instantaneous and induced dipoles, stronger id\u2013id forces, more energy needed to separate the molecules.<\/p>\n<h3>Explain why a branched isomer boils lower<\/h3><p>Same number of electrons, but a smaller surface area of contact between molecules, so weaker id\u2013id forces.<\/p>\n<h3>State the forces between molecules of a polar substance<\/h3><p>Permanent dipole\u2013permanent dipole (pd\u2013pd) forces in addition to id\u2013id forces; if the molecule has N-H, O-H or H-F, hydrogen bonds as well.<\/p>\n<h3>Do not say<\/h3><p>&#8220;The bonds break&#8221; for boiling; &#8220;id\u2013id forces only occur in non-polar molecules&#8221;; &#8220;the dipole is permanent&#8221;.<\/p>\n<\/article>\n\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the id\u2013id forces ideas that appear most often in Cambridge International A Level Chemistry questions.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>What are id\u2013id forces?<\/h3>\n<p>They are attractions between an instantaneous dipole in one molecule and the dipole it induces in a neighbouring molecule. The electrons in any particle are constantly moving, so at any instant the electron cloud can be uneven, and that temporary dipole induces an opposite dipole in the neighbour.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Do id\u2013id forces act between all molecules?<\/h3>\n<p>Yes. Every molecule and every separate atom has electrons, so every one of them has id\u2013id forces. A polar molecule has them in addition to its permanent dipole\u2013permanent dipole (pd\u2013pd) forces, never instead of them.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What makes them stronger?<\/h3>\n<p>Two things. The number of electrons is the larger factor: more electrons mean a bigger, more polarisable cloud and larger induced dipoles. The shape matters too, because long straight molecules touch their neighbours over a larger area than compact branched ones.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why does boiling point increase down Group 17?<\/h3>\n<p>The number of electrons per molecule rises from 18 in F\u2082 to 106 in I\u2082, so the id\u2013id forces between the molecules get stronger and more energy is needed to separate them. That is why iodine is a solid and chlorine a gas at room temperature.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Are van der Waals forces the same thing?<\/h3>\n<p>Cambridge uses van der Waals&#8217; forces as the family name for every intermolecular force other than a bond, and then divides the family into id\u2013id forces and pd\u2013pd forces, with hydrogen bonding as a special case of pd\u2013pd forces.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Intermolecular Forces Pages<\/h2>\n<p>Use these pages to connect the three intermolecular forces with the physical properties they explain.<\/p>\n<div class=\"ols-related-grid\">\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/permanent-dipole-dipole-forces\/\">Permanent Dipole-Dipole Forces<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/hydrogen-bonding\/\">Hydrogen Bonding<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/anomalous-properties-of-water\/\">Anomalous Properties of Water<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/boiling-temperature-trends\/\">Boiling Point Trends<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/choosing-solvents\/\">Choosing Solvents<\/a>\n<\/div>\n<\/section>\n<section class=\"ols-attribution-card\">\n        <p><strong>Copyright and author footprint:<\/strong> This OLS revision page was written for Online Learning System by <strong>Dr. Mohammed Al-Fatah<\/strong>. It is designed for A Level Chemistry revision and should not be copied or redistributed without permission.<\/p>\n      <\/section>\n\n      <div class=\"ols-image-lightbox\" id=\"olsImageLightboxNatureCovalentBonding9ch0\" aria-hidden=\"true\" role=\"dialog\" aria-modal=\"true\" aria-label=\"Expanded revision image\">\n        <div class=\"ols-image-lightbox-inner\">\n          <button class=\"ols-image-lightbox-close\" type=\"button\" aria-label=\"Close enlarged image\">\u00d7<\/button>\n          <img decoding=\"async\" class=\"ols-image-lightbox-img\" src=\"\" alt=\"\">\n        <\/div>\n      <\/div>\n\n      <script>\n        (function(){\n          var page = document.querySelector(\".ols-nature-of-covalent-bonding-9ch0-page\");\n          if (!page) { return; }\n          var lightbox = page.querySelector(\"#olsImageLightboxNatureCovalentBonding9ch0\");\n          if (!lightbox) { return; }\n          var lightboxImage = lightbox.querySelector(\".ols-image-lightbox-img\");\n    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