{"id":3115,"date":"2026-05-04T11:25:36","date_gmt":"2026-05-04T10:25:36","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/?page_id=3115"},"modified":"2026-10-19T20:07:46","modified_gmt":"2026-10-19T19:07:46","slug":"alkene-bonding","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/alkene-bonding\/","title":{"rendered":"Alkene 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-alkene-bonding-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: #c9973a;\n      --grey-text: #667085;\n      --body-text: #1f2937;\n      --border: rgba(28, 36, 75, 0.14);\n      --shadow: 0 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font-weight: 700; color: #1C244B; }\n  \n.ols-faq-card,\n    .ols-quicksnap-card {\n      background: #ffffff;\n      border: 1px solid var(--border);\n      border-radius: 28px;\n      box-shadow: var(--shadow);\n      padding: 34px;\n      margin-bottom: 24px;\n      overflow: hidden;\n    }\n.ols-faq-card h2,\n    .ols-quicksnap-card h2,\n    .ols-infographic-title {\n      margin: 0;\n      font-size: clamp(28px, 3.5vw, 42px);\n      line-height: 1.15;\n      font-weight: 800;\n      color: #111827;\n      letter-spacing: -0.03em;\n    }\n.ols-faq-card h2,\n    .ols-quicksnap-card h2 {\n      margin-bottom: 14px;\n    }\n.ols-faq-card p,\n    .ols-quicksnap-card p {\n      font-size: clamp(15px, 1.25vw, 17px);\n      line-height: 1.65;\n      font-weight: 300;\n      color: var(--body-text);\n    }\n.ols-faq-card strong,\n    .ols-quicksnap-card strong {\n      font-weight: 700;\n      color: var(--navy);\n    }\n.ols-faq-list {\n      display: grid;\n      gap: 14px;\n      margin-top: 18px;\n    }\n.ols-faq-item {\n      background: #ffffff;\n      border: 1px solid var(--border);\n      border-radius: 18px;\n      padding: 18px 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\">\r\n  <div class=\"ols-sidebar-header\">\r\n    <h3>Revision Notes<\/h3>\r\n    <p>A Level Chemistry<\/p>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Topic 5 Alkenes<\/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-5-alkenes\/alkene-bonding\/\">Alkene Bonding<\/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\/geometric-isomerism\/\">Geometric Isomerism<\/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\/electrophilic-addition-mechanism\/\">Electrophilic Addition Mechanism<\/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\/reactions-of-alkenes\/\">Reactions of Alkenes<\/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-5-alkenes\/reactions-of-alkenes\/halogens\/\">Halogens<\/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\/reactions-of-alkenes\/hydrogen-halide\/\">Hydrogen Halide<\/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\/reactions-of-alkenes\/bromine-water\/\">Bromine Water<\/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\/reactions-of-alkenes\/hydrogen\/\">Hydrogen<\/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\/reactions-of-alkenes\/steam\/\">Steam<\/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\/reactions-of-alkenes\/kmno4\/\">KMnO4<\/a>\r\n          <\/li>\r\n        <\/ul>\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\/polymerisation-reactions\/\">Polymerisation Reactions<\/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\/polymer-waste\/\">Polymer Waste<\/a>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Next Topics<\/h4>\r\n\r\n    <ul class=\"ols-topic-list\">\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/\">Energetics<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/\">Intermolecular Forces<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8A-redox-chemistry\/\">Redox Chemistry<\/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    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Chemistry<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Edexcel International<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/\">Topic 5 Alkenes<\/a> \/\n        <span>Alkene Bonding<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Alkene Bonding<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to alkene structure, carbon-carbon double bonds, sigma bonds, pi bonds and why alkenes are more reactive than alkanes.<\/p>\n        <div class=\"ols-badges\">\n          <div class=\"ols-badge\">Exam board: Edexcel International<\/div>\n          <div class=\"ols-badge\">Topic 5: Alkenes<\/div>\n        <\/div>\n      <\/header>\n<!-- 3D card: sigma-pi-bonds (N1, replaced old sigma + pi cards 29 Sep 2026) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, 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text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-sgpi-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-sgpi-001 a:hover{text-decoration:underline;}\n.ols-sgpi-001 .sgpi-seg button{display:inline-flex; align-items:center;}\n.ols-sgpi-001 .sgpi-seg button .sgpi-n{display:inline-flex; align-items:center; justify-content:center; width:20px; height:20px; margin-right:9px; border-radius:50%; font-size:11px; font-weight:600; background:rgba(37,99,235,0.10); color:#2563eb; flex:0 0 auto;}\n.ols-sgpi-001 .sgpi-seg button[aria-pressed=\"true\"] .sgpi-n{background:#ffffff; color:#2563eb;}\n<\/style>\n\n<div class=\"sgpi-head\">\n  <h2 class=\"sgpi-title\">Sigma and Pi Bonds: End-On and Sideways Overlap<\/h2>\n  <p class=\"sgpi-sub\">Choose a step to watch atoms approach, their orbitals combine into a \u03c3 molecular orbital and then a \u03c0 molecular orbital, and see why a C=C bond cannot twist.<\/p>\n<\/div>\n\n<div class=\"sgpi-stage\" id=\"sgpiStage\">\n  <canvas class=\"sgpi-canvas\" id=\"sgpiCanvas\"><\/canvas>\n  <div class=\"sgpi-overlay\" id=\"sgpiOverlay\"><\/div>\n  <div class=\"sgpi-caption\" id=\"sgpiCaption\"><\/div>\n  <div class=\"sgpi-hint\" id=\"sgpiHint\">Drag to rotate<\/div>\n  <div class=\"sgpi-badges\" id=\"sgpiBadges\"><\/div>\n<\/div>\n\n<div class=\"sgpi-controls\">\n  <div class=\"sgpi-row\">\n    <span class=\"sgpi-rowlab\">Step<\/span>\n    <div class=\"sgpi-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\"><span class=\"sgpi-n\">1<\/span>Two atoms approach<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\"><span class=\"sgpi-n\">2<\/span>\u03c3 MO forms<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\"><span class=\"sgpi-n\">3<\/span>\u03c0 MO forms<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\"><span class=\"sgpi-n\">4<\/span>The nodal plane<\/button>\n      <button type=\"button\" data-step=\"5\" aria-pressed=\"false\"><span class=\"sgpi-n\">5<\/span>No rotation about C=C<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"sgpi-row\">\n    <span class=\"sgpi-rowlab\">View<\/span>\n    <button type=\"button\" class=\"sgpi-pill\" id=\"sgpiResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"sgpi-info\">\n  <h3 id=\"sgpiInfoTitle\"><\/h3>\n  <p id=\"sgpiInfoText\"><\/p>\n  <div class=\"sgpi-facts\" id=\"sgpiFacts\"><\/div>\n  <div class=\"sgpi-key\">\n    <span><i class=\"k-ball\" style=\"background:#484e5c\"><\/i>Carbon<\/span>\n    <span><i class=\"k-ball\" style=\"background:#ececf0\"><\/i>Hydrogen<\/span>\n    <span><i class=\"k-ball\" style=\"background:#34aa48\"><\/i>Chlorine<\/span>\n    <span><i class=\"\" style=\"background:rgba(150,156,170,0.6)\"><\/i>1s orbital<\/span>\n    <span><i class=\"\" style=\"background:rgba(58,160,72,0.6)\"><\/i>3p orbital<\/span>\n    <span><i class=\"\" style=\"background:rgba(236,140,40,0.65)\"><\/i>p orbital<\/span>\n    <span><i class=\"\" style=\"background:rgba(240,188,40,0.75)\"><\/i>\u03c3 molecular orbital<\/span>\n    <span><i class=\"\" style=\"background:rgba(22,150,175,0.65)\"><\/i>\u03c0 molecular orbital<\/span>\n    <span><i class=\"\" style=\"background:rgba(220,50,50,0.14);border-color:rgba(200,40,40,0.7)\"><\/i>Nodal plane<\/span>\n  <\/div>\n<\/div>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/sigma-pi-bonds.js?v=20261018s\"><\/script>\n<\/section>\n\n<p class=\"ols-cc-sgpi-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n\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: Shared Pairs of Electrons<\/h2>\n<p>Three quick questions on single and double covalent bonds before you look inside the C=C bond.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"732\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">1<\/div><h2>What Makes an Alkene Unsaturated?<\/h2><\/div>\n        <p>Alkenes are hydrocarbons described as <strong>unsaturated<\/strong>. This means they do not contain the maximum possible number of hydrogen atoms.<\/p>\n        <p>The key structural feature of an alkene is at least one <strong>carbon-carbon double bond<\/strong>, written as <strong>C=C<\/strong>. For alkenes with one double bond and no rings, the general formula is <strong>C<sub>n<\/sub>H<sub>2n<\/sub><\/strong>.<\/p>\n        <p>Small alkenes include <strong>ethene<\/strong>, <strong>propene<\/strong> and <strong>butene<\/strong>. In longer alkenes, the position of the double bond can change, producing <strong>positional isomers<\/strong> such as but-1-ene and but-2-ene.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Key idea:<\/strong> An alkene must contain a C=C double bond, and this double bond is responsible for the characteristic chemistry of alkenes.<\/p><\/div>\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\" role=\"img\" aria-label=\"Examples of alkenes showing ethene, propene, but-1-ene and but-2-ene with the general formula CnH2n\" style=\"--ols-image: url('https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/alkenes-overview-positional-isomerism.webp');\"><\/div>\n          <div class=\"ols-figure-caption\">\n            <h3>Alkenes and positional isomers<\/h3>\n            <p>Shows unsaturation, the C=C bond, C<sub>n<\/sub>H<sub>2n<\/sub>, and how butene can exist as but-1-ene or but-2-ene.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Formulae and Positional Isomers<\/h2>\n<p>Apply the general formula to alkenes that are not named on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-3\" class=\"h5p-iframe\" data-content-id=\"3\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Alkene Bonding Quick-Fire: General Formula and Positional Isomers\"><\/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: A Ring as Well as a Double Bond<\/h2>\n<p>Work out how the general formula changes when the alkene is cyclic.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-16\" class=\"h5p-iframe\" data-content-id=\"16\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Alkene Bonding Drag: Formulae of Rings and Double Bonds\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">2<\/div><h2>The Carbon-Carbon Double Bond<\/h2><\/div>\n        <p>A carbon-carbon double bond is made from <strong>two different covalent bonds<\/strong>: one <strong>sigma, \u03c3, bond<\/strong> and one <strong>pi, \u03c0, bond<\/strong>.<\/p>\n        <p>The sigma bond forms first and lies directly between the two carbon nuclei. The pi bond forms from sideways overlap of p orbitals, giving electron density above and below the plane of the molecule.<\/p>\n        <p>The pi bond is more exposed than the sigma bond. This creates a region of <strong>high electron density<\/strong> that can attract electron-deficient species called <strong>electrophiles<\/strong>.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Exam focus:<\/strong> When explaining alkene reactivity, link the exposed pi bond to high electron density and attraction to electrophiles.<\/p><\/div>\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\" role=\"img\" aria-label=\"Diagram showing a carbon-carbon double bond made from one sigma bond and one pi bond\" style=\"--ols-image: url('https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/carbon-carbon-double-bond-sigma-pi-bond.webp');\"><\/div>\n          <div class=\"ols-figure-caption\">\n            <p>The C=C double bond contains one sigma bond and one pi bond, with the pi bond forming above and below the molecular plane.<\/p>\n          <\/div>\n        <\/div>\n      <div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-16-pireactive.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-16-pireactive.jpg\" alt=\"Two carbon nuclei with a \u03c3 bond between them and an exposed \u03c0 cloud above and below attracting an electrophile, plus a \u03c3 versus \u03c0 table\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-16-pireactive.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The \u03c0 bond holds exposed electron density above and below the plane, so electrophiles are attracted to this region of high electron density.<\/p><\/div>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Count the \u03c3 and \u03c0 Bonds<\/h2>\n<p>Count every bond in the molecule shown, remembering what each double bond contains.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-296\" class=\"h5p-iframe\" data-content-id=\"296\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Sigma and pi bonds in a cumulated diene molecule\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">3<\/div><h2>Formation of the Sigma Bond<\/h2><\/div>\n        <p>When two carbon atoms form a double bond, each carbon uses one <strong>sp<sup>2<\/sup> hybrid orbital<\/strong> to overlap directly with the other carbon atom.<\/p>\n        <p>This direct head-on overlap produces a strong <strong>carbon-carbon sigma bond<\/strong>. The electron density is concentrated between the two nuclei, so the sigma bond lies along the straight line joining the nuclei.<\/p>\n        <p>A single sigma bond allows free rotation around the bond axis. However, in a C=C double bond, this rotation is restricted because the pi bond must also be maintained.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Remember:<\/strong> Sigma bonds are formed by direct overlap along the internuclear axis.<\/p><\/div>\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\" role=\"img\" aria-label=\"Head-on orbital overlap forming a sigma bond between two carbon atoms\" style=\"--ols-image: url('https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/sigma-bond-formation-sp2-overlap.png.webp');\"><\/div>\n          <div class=\"ols-figure-caption\">\n            <p>Direct head-on overlap places electron density between the nuclei, forming the first bond in the carbon-carbon double bond.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Which Bond Can Rotate?<\/h2>\n<p>Decide where rotation is possible in two molecules that are not on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-9\" class=\"h5p-iframe\" data-content-id=\"9\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Alkene Bonding MCQ: Rotation in 1,2-Dichloroethane and 1,2-Dichloroethene\"><\/iframe><\/div><\/div>\n<\/section>\n\n      \n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">4<\/div><h2>Formation of the Pi Bond<\/h2><\/div>\n        <p>After the sigma bond forms, each carbon retains one <strong>unhybridised p orbital<\/strong> perpendicular to the plane of the molecule.<\/p>\n        <p>The <strong>pi, \u03c0, bond<\/strong> forms when these p orbitals overlap sideways. This produces regions of electron density above and below the line joining the two carbon nuclei.<\/p>\n        <p>Because sideways overlap is less direct than sigma overlap, the pi bond is weaker than the sigma bond. The pi bond also prevents free rotation around the C=C bond.<\/p>\n        <div class=\"ols-definition-box\">\n          <div class=\"ols-definition-circle\">\u03c0<\/div>\n          <p><strong>Pi bond:<\/strong> A covalent bond formed by sideways overlap of p orbitals, with electron density above and below the internuclear axis.<\/p>\n        <\/div>\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\" role=\"img\" aria-label=\"Sideways overlap of p orbitals forming a pi bond above and below the carbon-carbon bond axis\" style=\"--ols-image: url('https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/pi-bond-formation-p-orbital-overlap.png.webp');\"><\/div>\n          <div class=\"ols-figure-caption\">\n            <h3>Formation of the \u03c0 bond<\/h3>\n            <p>Sideways p-orbital overlap produces electron density above and below the C-C bond axis, restricting rotation and increasing reactivity.<\/p>\n          <\/div>\n        <\/div>\n\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. All rights reserved. -->\n<!-- 3D card: hybridisation-carbon (rebuilt Oct 2026) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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height:20px; margin-right:9px; border-radius:50%; font-size:11px; font-weight:600; background:rgba(37,99,235,0.10); color:#2563eb; flex:0 0 auto;}\n.ols-hybc-001 .hybc-seg button[aria-pressed=\"true\"] .hybc-n{background:#ffffff; color:#2563eb;}\n<\/style>\n\n<div class=\"hybc-head\">\n  <h2 class=\"hybc-title\">Hybridisation of Carbon: sp\u00b3, sp\u00b2 and sp Orbitals<\/h2>\n  <p class=\"hybc-sub\">Choose a step to watch carbon\u2019s 2s and 2p orbitals mix into sp\u00b3, sp\u00b2 or sp hybrids, see the angle between them and which 2p orbitals are left for \u03c0 bonds.<\/p>\n<\/div>\n\n<div class=\"hybc-stage\" id=\"hybcStage\">\n  <canvas class=\"hybc-canvas\" id=\"hybcCanvas\"><\/canvas>\n  <div class=\"hybc-overlay\" id=\"hybcOverlay\"><\/div>\n  <div class=\"hybc-caption\" id=\"hybcCaption\"><\/div>\n  <div class=\"hybc-hint\" id=\"hybcHint\">Drag to rotate<\/div>\n  <div class=\"hybc-badges\" id=\"hybcBadges\"><\/div>\n<\/div>\n\n<div class=\"hybc-controls\">\n  <div class=\"hybc-row\">\n    <span class=\"hybc-rowlab\">Step<\/span>\n    <div class=\"hybc-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\"><span class=\"hybc-n\">1<\/span>The 2s and 2p orbitals<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\"><span class=\"hybc-n\">2<\/span>sp\u00b3: four hybrids<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\"><span class=\"hybc-n\">3<\/span>sp\u00b2: three hybrids<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\"><span class=\"hybc-n\">4<\/span>sp: two hybrids<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"hybc-row\">\n    <span class=\"hybc-rowlab\">View<\/span>\n    <button type=\"button\" class=\"hybc-pill\" id=\"hybcResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"hybc-info\">\n  <h3 id=\"hybcInfoTitle\"><\/h3>\n  <p id=\"hybcInfoText\"><\/p>\n  <div class=\"hybc-facts\" id=\"hybcFacts\"><\/div>\n  <div class=\"hybc-key\">\n    <span><i class=\"k-ball\" style=\"background:#464a54\"><\/i>Carbon nucleus<\/span>\n    <span><i class=\"k-ball\" style=\"background:#ececf0\"><\/i>Hydrogen nucleus<\/span>\n    <span><i class=\"k-ball\" style=\"background:#facc28\"><\/i>Electron<\/span>\n    <span><i class=\"\" style=\"background:rgba(150,156,170,0.6)\"><\/i>2s orbital<\/span>\n    <span><i class=\"\" style=\"background:rgba(236,140,40,0.6)\"><\/i>2p orbital (unhybridised)<\/span>\n    <span><i class=\"\" style=\"background:rgba(150,90,200,0.6)\"><\/i>Second unhybridised 2p<\/span>\n    <span><i class=\"\" style=\"background:rgba(70,120,215,0.6)\"><\/i>Hybrid orbital<\/span>\n    <span><i class=\"\" style=\"background:rgba(240,188,40,0.75)\"><\/i>\u03c3 molecular orbital (C\u2013H bond)<\/span>\n  <\/div>\n<\/div>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/hybridisation-carbon.js?v=20261019d\"><\/script>\n<\/section>\n\n<p class=\"ols-cc-hybc-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n<p class=\"ols-hyb-001-cc\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n      <div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-17-bondenthalpy.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-17-bondenthalpy.jpg\" alt=\"Bar chart of mean bond enthalpies: C\u2013C 347, C=C 612 and a dotted bar at 694 kJ\/mol, showing the \u03c0 bond adds only about 265\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-17-bondenthalpy.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The C=C bond (612 kJ mol\u207b\u00b9) is less than twice the C\u2013C bond (347 kJ mol\u207b\u00b9), so the \u03c0 bond adds only about 265 kJ mol\u207b\u00b9 and breaks more easily.<\/p><\/div>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Explain the Bond Enthalpy Data<\/h2>\n<p>Write a short explanation, then compare it with the mark points and the model answer.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-10\" class=\"h5p-iframe\" data-content-id=\"10\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Alkene Bonding Explain: Why C=C Is Not Twice as Strong as C-C\"><\/iframe><\/div><\/div>\n<\/section>\n\n      \n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">5<\/div><h2>Comparing Sigma and Pi Bonds<\/h2><\/div>\n        <p>Exam questions often test whether you can distinguish sigma bonding from pi bonding and connect this bonding to alkene reactivity.<\/p>\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr><th>Feature<\/th><th>Sigma bond<\/th><th>Pi bond<\/th><\/tr>\n            <\/thead>\n            <tbody>\n              <tr><td><strong>How it forms<\/strong><\/td><td>Direct head-on orbital overlap<\/td><td>Sideways overlap of p orbitals<\/td><\/tr>\n              <tr><td><strong>Electron density<\/strong><\/td><td>Between the two nuclei<\/td><td>Above and below the molecular plane<\/td><\/tr>\n              <tr><td><strong>Effect in alkenes<\/strong><\/td><td>Forms the first bond between the carbon atoms<\/td><td>Creates an exposed electron-rich region that reacts with electrophiles<\/td><\/tr>\n            <\/tbody>\n          <\/table>\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: \u03c3 and \u03c0 Bonds in Propene<\/h2>\n<p>In each round, pick the one statement about propene that is accurate.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-733\" class=\"h5p-iframe\" data-content-id=\"733\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Alkene Bonding Summary: \u03c3 and \u03c0 Bonds in Propene\"><\/iframe><\/div><\/div>\n<\/section>\n\n      \n\n      <section class=\"ols-infographic-full\" aria-label=\"Alkene bonding visual summary\">\n        <div class=\"ols-infographic-full-frame\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-mv-01-displayed.svg\" alt=\"Displayed formulae of ethene, propene, but-1-ene, but-2-ene\" width=\"1433\" height=\"1157\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-cd-01-ccsigma.svg\" alt=\"\u03c3 molecular orbital between the carbon nuclei of ethene, and \u03c0 orbital above and below the plane of the molecule\" width=\"1514\" height=\"1042\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-cd-01-ccpi.svg\" alt=\"\u03c3 molecular orbital between the carbon nuclei of ethene, and \u03c0 orbital above and below the plane of the molecule\" width=\"1508\" height=\"972\">\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>These questions secure the bonding ideas that every alkene reaction and mechanism depends on.<\/p>\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why is the \u03c0 bond weaker than the \u03c3 bond when both are covalent bonds?<\/h3>\n<p>The \u03c0 bond forms by sideways overlap of p orbitals, which is less effective than the head-on overlap of the \u03c3 bond. Less overlap gives a lower bond enthalpy, so the \u03c0 bond is the bond that breaks in addition reactions.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Is a C=C bond twice as strong as a C\u2013C single bond?<\/h3>\n<p>No. The C=C bond is stronger than a C\u2013C bond but less than twice as strong, because the \u03c0 bond is weaker than the \u03c3 bond. Mean bond enthalpies are about 612 kJ mol<sup>\u22121<\/sup> for C=C and 347 kJ mol<sup>\u22121<\/sup> for C\u2013C.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Does every hydrocarbon with the formula C<sub>n<\/sub>H<sub>2n<\/sub> contain a C=C bond?<\/h3>\n<p>No. Cyclohexane, C<sub>6<\/sub>H<sub>12<\/sub>, fits C<sub>n<\/sub>H<sub>2n<\/sub> but has a ring and no double bond. Cyclohexene, C<sub>6<\/sub>H<sub>10<\/sub>, has a ring and a C=C bond. A formula alone cannot prove that a compound is an alkene.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Why can a C=C bond not rotate when a C\u2013C bond can?<\/h3>\n<p>Rotating one carbon of a C=C bond would twist the two p orbitals out of line and break the sideways overlap of the \u03c0 bond. A \u03c3 bond keeps its overlap as it rotates, so single bonds rotate freely.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Is the \u03c0 bond two bonds, one above and one below the molecule?<\/h3>\n<p>No. The \u03c0 bond has two regions of electron density, above and below the plane of the molecule, but together they make one bond holding two electrons. A C=C bond is one \u03c3 bond plus one \u03c0 bond.<\/p>\n<\/div>\n<\/div>\n<\/section>\n\n      <!-- author card (moved to the end, 10 Oct 2026) -->\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\n        class=\"ols-linkedin-pill\"\n        href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\"\n        target=\"_blank\"\n        rel=\"noopener noreferrer\"\n      >\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\n\n      <section class=\"ols-attribution-card ols-attr-std\" aria-label=\"Copyright notice\">\n<style id=\"ols-attribution-card-css\">.ols-attribution-card.ols-attr-std{background:linear-gradient(135deg,#ffffff,#f8fbff);border:1px solid rgba(28,36,75,0.14);border-radius:28px;box-shadow:0 18px 45px rgba(28,36,75,0.10);padding:34px;margin:24px 0;overflow:hidden;font-family:Poppins,Arial,sans-serif;box-sizing:border-box}.ols-attribution-card.ols-attr-std *{box-sizing:border-box}.ols-attribution-card.ols-attr-std p{margin:0;font-size:14px;line-height:1.65;font-weight:300;color:#667085}.ols-attribution-card.ols-attr-std strong{font-weight:700;color:#1C244B}@media (max-width:760px){.ols-attribution-card.ols-attr-std{padding:24px 18px;border-radius:22px}}<\/style>\n<p><strong>Copyright notice:<\/strong> This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.<\/p>\n<\/section>\n    <\/main>\n  \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-5-alkenes\/alkene-bonding\/#webpage\",\n            \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/alkene-bonding\/\",\n            \"name\": \"Alkene Bonding | Topic 5 Alkenes | Online Learning System\",\n            \"description\": \"A Level Chemistry revision notes covering alkene bonding, sigma bonds, pi bonds, orbital overlap and alkene reactivity for Edexcel International A Level Chemistry.\",\n            \"isPartOf\": {\n                \"@id\": 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