{"id":8172,"date":"2026-09-16T17:33:59","date_gmt":"2026-09-16T16:33:59","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/hydrogen-bonding\/"},"modified":"2026-09-22T09:59:27","modified_gmt":"2026-09-22T08:59:27","slug":"hydrogen-bonding","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/intermolecular-forces\/hydrogen-bonding\/","title":{"rendered":"Hydrogen Bonding"},"content":{"rendered":"<script src=\"https:\/\/cdnjs.cloudflare.com\/ajax\/libs\/three.js\/r128\/three.min.js\"><\/script>\n\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      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transparent; }\n    .ols-figure-placeholder { background: #ffffff; border: 1px solid rgba(28, 36, 75, 0.12); border-radius: 22px; padding: 14px; margin: 18px 0 6px; }\n    .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>Hydrogen Bonding<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Hydrogen Bonding<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to hydrogen bonding: the N-H, O-H and H-F requirement, the four features of the exam diagram, hydrogen bonding in water, ammonia and hydrogen fluoride, and predicting it in other molecules, 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: Separate Molecules and What Boiling Breaks<\/h2>\n<p>Three quick questions on simple molecules from GCSE before you meet the strongest force that acts between them.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"781\"><\/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 a Hydrogen Bond Is<\/h2>\n<\/div>\n<p>A <strong>hydrogen bond<\/strong> is a strong intermolecular attraction between a hydrogen atom that is covalently bonded to nitrogen, oxygen or fluorine in one molecule and a <strong>lone pair<\/strong> of electrons on a nitrogen, oxygen or fluorine atom in a neighbouring molecule. It is the strongest of the three intermolecular forces, but it is still far weaker than a covalent bond.<\/p>\n<p>Three things make the bond possible. Nitrogen, oxygen and fluorine are the three most electronegative elements, so the H atom bonded to one of them carries a large \u03b4+ charge. The hydrogen atom is tiny and has no inner electrons, so its \u03b4+ charge is concentrated and can approach a neighbouring atom very closely. The N, O or F atom on the neighbour has a lone pair that points its negative charge directly at the \u03b4+ hydrogen. The result is a directional attraction, with the X-H\u00b7\u00b7\u00b7Y atoms in a straight line, that is much stronger than an ordinary permanent dipole\u2013permanent dipole (pd\u2013pd) attraction.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Definition:<\/strong> A hydrogen bond forms between a \u03b4+ hydrogen atom bonded to N, O or F and a lone pair on N, O or F in another molecule.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>The Exam Diagram<\/h2>\n<\/div>\n<p>Questions that ask you to <em>show<\/em> hydrogen bonding are marked on four features, and a diagram that misses any of them loses marks:<\/p>\n<ol>\n<li>The <strong>lone pair<\/strong> drawn on the N, O or F atom that accepts the bond.<\/li>\n<li><strong>\u03b4+<\/strong> on the hydrogen and <strong>\u03b4-<\/strong> on the electronegative atom in both molecules.<\/li>\n<li>The hydrogen bond drawn as a <strong>dashed line<\/strong> from the \u03b4+ H to the lone pair, and labelled.<\/li>\n<li>The three atoms X-H\u00b7\u00b7\u00b7Y in a <strong>straight line<\/strong> (a 180\u00b0 arrangement), even though the molecules themselves are bent.<\/li>\n<\/ol>\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\/hydrogen-bonding-in-water-ammonia-and-hydrogen-fluoride.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\/hydrogen-bonding-in-water-ammonia-and-hydrogen-fluoride.jpg\" alt=\"Exam-style hydrogen bond diagrams for water, ammonia and hydrogen fluoride with lone pairs, partial charges and dashed hydrogen bonds\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The four marked features of a hydrogen bond diagram: lone pair, \u03b4 charges, a labelled dashed line and a straight X-H\u00b7\u00b7\u00b7Y arrangement.<\/p><\/div>\n<\/div>\n<p>The same four features are marked when the two molecules are <strong>different<\/strong>. For ammonia with water, the dashed line runs either from a \u03b4+ hydrogen of the water to the lone pair on the nitrogen, or from a \u03b4+ hydrogen of the ammonia to a lone pair on the oxygen. Either direction earns the mark, provided the lone pair is drawn and the three atoms are in line.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam trap:<\/strong> The dashed line must start at the hydrogen atom and end at the lone pair, not at the O, N or F atom in general. Draw the lone pair first, then the line to it.<\/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: Marking a Diagram<\/h2>\n<p>Apply the four marked features to a diagram of two ethanol molecules and find the one that would lose a mark.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-532\" class=\"h5p-iframe\" data-content-id=\"532\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogen Bonding MCQ: Marking a Diagram of Two Ethanol Molecules\"><\/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>Hydrogen Bonding in Water<\/h2>\n<\/div>\n<p>Each water molecule has two O-H bonds and two lone pairs on the oxygen, so each molecule can form up to <strong>four<\/strong> hydrogen bonds: two through its own hydrogen atoms and two through its lone pairs. In <strong>ice<\/strong> all four are made, because the network is fixed; in <strong>liquid water<\/strong> the network is constantly changing and each molecule averages about <strong>3.5<\/strong> hydrogen bonds at any moment. The ice network is an open lattice (see the <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> page).<\/p>\n<section class=\"ols-hb-001\" id=\"olsHb001\">\n<style>\n@import url('https:\/\/fonts.googleapis.com\/css2?family=Poppins:wght@300;400;500;600&display=swap');\n\n.ols-hb-001{\n  font-family:'Poppins',system-ui,-apple-system,'Segoe UI',Roboto,Helvetica,Arial,sans-serif;\n  color:#1C244B; background:#ffffff;\n  border:1px solid rgba(28,36,75,0.14);\n  border-radius:28px;\n  box-shadow:0 18px 45px rgba(28,36,75,0.10);\n  padding:44px; margin:26px auto; max-width:980px;\n  box-sizing:border-box; -webkit-font-smoothing:antialiased;\n}\n.ols-hb-001 *{box-sizing:border-box;}\n\n.ols-hb-001 .hb-head{margin:0 0 22px;}\n.ols-hb-001 h2.hb-title{\n  font-size:26px; line-height:1.25; font-weight:600; letter-spacing:-0.012em;\n  margin:0 0 9px; 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padding:4px 8px;}\n  .ols-hb-001 .hb-lab.l-ang{font-size:12px;}\n  .ols-hb-001 .hb-caption{font-size:10.5px; padding:5px 9px; max-width:66%;}\n  .ols-hb-001 .hb-info{padding:18px;}\n}\n@media (prefers-reduced-motion:reduce){\n  .ols-hb-001 .hb-seg button,.ols-hb-001 .hb-pill{transition:none;}\n}\n\n.ols-cc-hb-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-hb-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-hb-001 a:hover{text-decoration:underline;}\n<\/style>\n\n<div class=\"hb-head\">\n  <h2 class=\"hb-title\">Hydrogen Bonding in Water: Four Neighbours in a Tetrahedral Arrangement<\/h2>\n  <p class=\"hb-sub\">Drag to rotate, scroll or pinch to zoom. Start with one water molecule and its two lone pairs, then add the four neighbours it can hydrogen bond to. The four bonds point to the corners of a tetrahedron.<\/p>\n<\/div>\n\n<div class=\"hb-stage\" id=\"hbStage\">\n  <canvas class=\"hb-canvas\" id=\"hbCanvas\"><\/canvas>\n  <div class=\"hb-overlay\" id=\"hbOverlay\"><\/div>\n  <div class=\"hb-caption\" id=\"hbCaption\"><\/div>\n  <div class=\"hb-hint\" id=\"hbHint\">Drag to rotate<\/div>\n<\/div>\n\n<div class=\"hb-controls\">\n  <div class=\"hb-row\">\n    <span class=\"hb-rowlab\">Show<\/span>\n    <div class=\"hb-seg\" role=\"group\" aria-label=\"What to show\">\n      <button type=\"button\" data-s=\"one\" aria-pressed=\"true\">One molecule<\/button>\n      <button type=\"button\" data-s=\"four\" aria-pressed=\"false\">Four neighbours<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"hb-row\">\n    <span class=\"hb-rowlab\">View<\/span>\n    <div class=\"hb-seg\" role=\"group\" aria-label=\"Camera view\">\n      <button type=\"button\" data-v=\"front\" aria-pressed=\"false\">Front<\/button>\n      <button type=\"button\" data-v=\"corner\" aria-pressed=\"false\">Corner<\/button>\n      <button type=\"button\" data-v=\"top\" aria-pressed=\"false\">Top<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"hb-row\">\n    <span class=\"hb-rowlab\">Detail<\/span>\n    <button type=\"button\" class=\"hb-pill p-rose\" id=\"hbTogLp\" aria-pressed=\"true\"><i class=\"dot\"><\/i>Lone pairs<\/button>\n    <button type=\"button\" class=\"hb-pill\" id=\"hbTogD\" aria-pressed=\"true\"><i class=\"dot\"><\/i>&delta; charges<\/button>\n    <button type=\"button\" class=\"hb-pill\" id=\"hbTogA\" aria-pressed=\"true\"><i class=\"dot\"><\/i>Angles<\/button>\n    <button type=\"button\" class=\"hb-pill\" id=\"hbTogL\" aria-pressed=\"true\"><i class=\"dot\"><\/i>Labels<\/button>\n  <\/div>\n\n  <div class=\"hb-row\">\n    <span class=\"hb-rowlab\">Motion<\/span>\n    <button type=\"button\" class=\"hb-pill\" id=\"hbSpin\" aria-pressed=\"false\"><i class=\"dot\"><\/i>Rotation<\/button>\n    <button type=\"button\" class=\"hb-pill\" id=\"hbResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"hb-info\">\n  <h3 id=\"hbInfoTitle\"><\/h3>\n  <p id=\"hbInfoText\"><\/p>\n  <div class=\"hb-facts\" id=\"hbFacts\"><\/div>\n  <div class=\"hb-key\">\n    <span><i style=\"background:rgba(198,46,40,0.85)\"><\/i>Oxygen, &delta;&minus;<\/span>\n    <span><i style=\"background:rgba(226,229,238,0.95)\"><\/i>Hydrogen, &delta;+<\/span>\n    <span><i style=\"background:rgba(198,62,122,0.5)\"><\/i>Lone pair<\/span>\n    <span><i style=\"background:rgba(37,99,235,0.55)\"><\/i>Hydrogen bond<\/span>\n  <\/div>\n<\/div>\n<\/section>\n\n<p class=\"ols-cc-hb-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\/water-hydrogen-bonding.js\"><\/script>\n<p>Because water forms so many hydrogen bonds per molecule, its boiling point is far higher than that of any hydride of similar size, and it takes a great deal of energy to boil.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Two O-H bonds plus two lone pairs give water the maximum of four hydrogen bonds per molecule; this is the root of every anomalous property of water.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Hydrogen Bonding in Ammonia and Hydrogen Fluoride<\/h2>\n<\/div>\n<p><strong>Ammonia, NH\u2083,<\/strong> has three N-H bonds but only <strong>one lone pair<\/strong> on nitrogen. Each molecule can therefore be involved in hydrogen bonds through its hydrogens, but only one neighbour at a time can bond to its lone pair, so on average liquid ammonia has about one hydrogen bond per molecule. Nitrogen is also less electronegative than oxygen, so the N-H hydrogen is less \u03b4+ and the bond is weaker. Ammonia boils at -33 \u00b0C.<\/p>\n<p><strong>Hydrogen fluoride, HF,<\/strong> has three lone pairs on fluorine but only <strong>one hydrogen<\/strong>, so again each molecule averages one hydrogen bond, forming zig-zag chains of molecules. Fluorine is the most electronegative element, so each individual F-H\u00b7\u00b7\u00b7F bond is the strongest hydrogen bond of all, and HF boils at 20 \u00b0C, far above HCl at -85 \u00b0C.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Liquid<\/th><th>H atoms on N, O or F<\/th><th>Lone pairs<\/th><th>Hydrogen bonds per molecule (average)<\/th><th>boiling point \/ \u00b0C<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>H\u2082O<\/strong><\/td><td>2<\/td><td>2<\/td><td>up to 4, about 3.5 in the liquid<\/td><td>100<\/td><\/tr>\n<tr><td><strong>HF<\/strong><\/td><td>1<\/td><td>3<\/td><td>about 1 (chains)<\/td><td>20<\/td><\/tr>\n<tr><td><strong>NH\u2083<\/strong><\/td><td>3<\/td><td>1<\/td><td>about 1<\/td><td>-33<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The order of boiling points, H\u2082O > HF > NH\u2083, follows the number of hydrogen bonds each molecule can make more closely than the strength of each bond. The <strong>number<\/strong> of hydrogen bonds is limited by whichever is fewer: suitable hydrogens or lone pairs.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> Water has a higher boiling point than ammonia because each water molecule can form more hydrogen bonds (two O-H bonds and two lone pairs) and O-H\u00b7\u00b7\u00b7O hydrogen bonds are stronger than N-H\u00b7\u00b7\u00b7N bonds, so more energy is needed to separate the 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: Counting Hydrogen Bonds<\/h2>\n<p>Use the rule that whichever is fewer, suitable hydrogens or lone pairs, sets the number of hydrogen bonds per molecule.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-782\" class=\"h5p-iframe\" data-content-id=\"782\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogen Bonding Rapid Check: Counting Hydrogen Bonds per Molecule\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Explain a Boiling Temperature<\/h2>\n<p>Write a short explanation for two alcohols with the same number of electrons but a 100 \u00b0C difference in boiling temperature.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-783\" class=\"h5p-iframe\" data-content-id=\"783\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogen Bonding Explain: Ethane-1,2-diol Against Propan-1-ol\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Predicting Hydrogen Bonding in Other Molecules<\/h2>\n<\/div>\n<p>The test is simple and should be applied literally: <strong>is there a hydrogen atom bonded directly to N, O or F?<\/strong> If yes, the substance forms hydrogen bonds between its molecules. If the only N, O or F atoms are bonded to carbon, the molecule may accept a hydrogen bond from water but cannot form one with its own kind.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Molecule<\/th><th>H on N, O or F?<\/th><th>Hydrogen bonds between its own molecules?<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Methanol, CH\u2083OH<\/strong><\/td><td>yes (O-H)<\/td><td>yes<\/td><\/tr>\n<tr><td><strong>Methylamine, CH\u2083NH\u2082<\/strong><\/td><td>yes (N-H)<\/td><td>yes<\/td><\/tr>\n<tr><td><strong>Ethanoic acid, CH\u2083COOH<\/strong><\/td><td>yes (O-H)<\/td><td>yes, in pairs (dimers)<\/td><\/tr>\n<tr><td><strong>Fluoromethane, CH\u2083F<\/strong><\/td><td>no (F on C, H on C)<\/td><td>no<\/td><\/tr>\n<tr><td><strong>Methoxymethane, CH\u2083OCH\u2083<\/strong><\/td><td>no<\/td><td>no, but accepts from water<\/td><\/tr>\n<tr><td><strong>Ethanal, CH\u2083CHO<\/strong><\/td><td>no<\/td><td>no, but accepts from water<\/td><\/tr>\n<tr><td><strong>Hydrogen chloride, HCl<\/strong><\/td><td>no (Cl is not N, O or F)<\/td><td>no<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>A good <strong>acceptor<\/strong> is a lone pair on a small, very electronegative atom that is not already committed elsewhere. Lone pairs on oxygen and nitrogen accept readily, which is why methoxymethane and ethanal accept from water. Fluorine bonded to carbon holds its lone pairs far too tightly, so CH\u2083F is a very poor acceptor and is listed above as forming no hydrogen bonds at all. An ion such as NH\u2084\u207a has no lone pair left on its nitrogen, so it can donate but not accept.<\/p>\n<p>Molecules of biological importance rely on this bond: the two strands of DNA are held together by hydrogen bonds between base pairs, proteins fold because of hydrogen bonds between N-H and C=O groups, and the hydroxyl groups of sugars make them soluble in water.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Common mistake:<\/strong> Assuming any molecule that contains oxygen forms hydrogen bonds. Propanone and ethers contain oxygen but no O-H, so they do not hydrogen bond with each other.<\/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 Molecules Really Hydrogen Bond<\/h2>\n<p>Pick the one accurate statement in each round about four molecules that catch students out.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-531\" class=\"h5p-iframe\" data-content-id=\"531\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogen Bonding Summary: Four Molecules Students Get Wrong\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Hydrogen Bonds Compared With Other Forces<\/h2>\n<\/div>\n<p>A hydrogen bond typically needs 10 to 40 kJ mol\u207b\u00b9 to break, roughly ten times a small permanent dipole\u2013permanent dipole (pd\u2013pd) attraction and a tenth of a covalent bond. It is still an <strong>intermolecular<\/strong> force: when water boils, hydrogen bonds break and the O-H covalent bonds do not.<\/p>\n<p>Hydrogen bonds act in addition to permanent dipole\u2013permanent dipole (pd\u2013pd) forces and id\u2013id forces, so a hydrogen-bonded liquid has all three forces between its molecules. In a small molecule such as water the hydrogen bonds dominate; in a large molecule such as hexan-1-ol the id\u2013id forces along the carbon chain contribute as much or more.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> Call it a hydrogen bond, not a &#8220;hydrogen bonding force&#8221; or a &#8220;covalent hydrogen bond&#8221;. State that it is intermolecular.<\/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: Two Molecules With the Same Formula<\/h2>\n<p>Build the comparison between two isomers, one of which can form hydrogen bonds and one of which cannot.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-530\" class=\"h5p-iframe\" data-content-id=\"530\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogen Bonding Drag: Propan-1-ol Against Methoxyethane\"><\/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>Draw hydrogen bonding between two molecules of X<\/h3><p>Lone pair on N, O or F; \u03b4+ and \u03b4-; dashed line labelled hydrogen bond from H to the lone pair; straight X-H\u00b7\u00b7\u00b7Y arrangement.<\/p>\n<h3>Explain why HF boils higher than HCl<\/h3><p>HF forms hydrogen bonds (F is highly electronegative and has lone pairs); HCl cannot; hydrogen bonds are stronger than the permanent dipole\u2013permanent dipole (pd\u2013pd) forces in HCl, so more energy is needed.<\/p>\n<h3>Does molecule X form hydrogen bonds?<\/h3><p>Only if it has H bonded directly to N, O or F. Oxygen bonded to carbon alone is not enough.<\/p>\n<h3>Do not say<\/h3><p>&#8220;Hydrogen bonds are covalent&#8221;; &#8220;hydrogen bonds form between hydrogen atoms&#8221;; &#8220;the O-H bond breaks when water boils&#8221;.<\/p>\n<\/article>\n\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the hydrogen bonding rules and the diagram marks in Cambridge International A Level Chemistry questions.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>When does hydrogen bonding occur?<\/h3>\n<p>Only when a hydrogen atom is bonded directly to nitrogen, oxygen or fluorine, and there is a lone pair on an N, O or F atom in a neighbouring molecule to accept it. Those three elements are electronegative enough to leave the hydrogen strongly \u03b4+.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What must a hydrogen bond diagram show?<\/h3>\n<p>Four things: the lone pair on the accepting atom, \u03b4+ and \u03b4- labels, a dashed line drawn from the \u03b4+ hydrogen to that lone pair, and the three atoms X-H\u00b7\u00b7\u00b7Y in a straight line.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How many hydrogen bonds can one water molecule form?<\/h3>\n<p>Four: two through its own hydrogen atoms and two through the two lone pairs on its oxygen. This is more than ammonia or hydrogen fluoride can manage and explains why water is the most anomalous of the three.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Does a molecule that contains oxygen always hydrogen bond?<\/h3>\n<p>No. Propanone and methoxymethane contain oxygen but have no O-H group, so their molecules cannot hydrogen bond with each other. They can still accept a hydrogen bond from water, which is why they dissolve in it.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Is a hydrogen bond a covalent bond?<\/h3>\n<p>No. It is an intermolecular force, roughly a tenth the strength of a covalent bond. When water boils the hydrogen bonds break and every O-H covalent bond stays intact.<\/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\/london-forces\/\">London 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\/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\/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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