{"id":3146,"date":"2026-05-04T17:47:48","date_gmt":"2026-05-04T16:47:48","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/?page_id=3146"},"modified":"2026-10-19T20:07:39","modified_gmt":"2026-10-19T19:07:39","slug":"geometric-isomerism","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/geometric-isomerism\/","title":{"rendered":"Geometric Isomerism"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-geometric-isomerism-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 18px 45px rgba(28, 36, 75, 0.10);\n      --inner-shadow: 0 10px 26px rgba(28, 36, 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    line-height: 1.65;\n      font-weight: 300;\n      color: #667085;\n    }\n    .ols-attribution-card strong { 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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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>Geometric Isomerism<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Geometric Isomerism<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to stereoisomerism in alkenes, restricted rotation around C=C bonds, E-Z naming, priority groups and the link between E-Z and cis-trans isomerism.<\/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<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-ezrt-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-ezrt-001 a:hover{text-decoration:underline;}\n.ols-ezrt-001 .ezrt-seg button{display:inline-flex; align-items:center;}\n.ols-ezrt-001 .ezrt-seg button .ezrt-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-ezrt-001 .ezrt-seg button[aria-pressed=\"true\"] .ezrt-n{background:#ffffff; color:#2563eb;}\n<\/style>\n\n<div class=\"ezrt-head\">\n  <h2 class=\"ezrt-title\">Free and Restricted Rotation: Why E\/Z Isomers Exist<\/h2>\n  <p class=\"ezrt-sub\">Choose a step to watch butane turn freely about its C\u2013C \u03c3 bond, then see how the \u03c0 bond locks but-2-ene into separate Z and E isomers.<\/p>\n<\/div>\n\n<div class=\"ezrt-stage\" id=\"ezrtStage\">\n  <canvas class=\"ezrt-canvas\" id=\"ezrtCanvas\"><\/canvas>\n  <div class=\"ezrt-overlay\" id=\"ezrtOverlay\"><\/div>\n  <div class=\"ezrt-caption\" id=\"ezrtCaption\"><\/div>\n  <div class=\"ezrt-hint\" id=\"ezrtHint\">Drag to rotate<\/div>\n  <div class=\"ezrt-badges\" id=\"ezrtBadges\"><\/div>\n<\/div>\n\n<div class=\"ezrt-controls\">\n  <div class=\"ezrt-row\">\n    <span class=\"ezrt-rowlab\">Step<\/span>\n    <div class=\"ezrt-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\"><span class=\"ezrt-n\">1<\/span>Free rotation in butane<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\"><span class=\"ezrt-n\">2<\/span>The \u03c0 bond locks C=C<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\"><span class=\"ezrt-n\">3<\/span>Z isomer<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\"><span class=\"ezrt-n\">4<\/span>E isomer<\/button>\n      <button type=\"button\" data-step=\"5\" aria-pressed=\"false\"><span class=\"ezrt-n\">5<\/span>Breaking the \u03c0 bond<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"ezrt-row\">\n    <span class=\"ezrt-rowlab\">View<\/span>\n    <button type=\"button\" class=\"ezrt-pill\" id=\"ezrtResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"ezrt-info\">\n  <h3 id=\"ezrtInfoTitle\"><\/h3>\n  <p id=\"ezrtInfoText\"><\/p>\n  <div class=\"ezrt-facts\" id=\"ezrtFacts\"><\/div>\n  <div class=\"ezrt-key\">\n    <span><i class=\"k-ball\" style=\"background:#464a54\"><\/i>Carbon<\/span>\n    <span><i class=\"k-ball\" style=\"background:#ececf0\"><\/i>Hydrogen<\/span>\n    <span><i class=\"\" style=\"background:rgba(70,120,215,0.6)\"><\/i>sp\u00b3 hybrid orbital<\/span>\n    <span><i class=\"\" style=\"background:rgba(236,140,40,0.6)\"><\/i>p orbital<\/span>\n    <span><i class=\"\" style=\"background:rgba(226,150,30,0.6)\"><\/i>\u03c3 molecular orbital (replaces two orbitals)<\/span>\n    <span><i class=\"\" style=\"background:rgba(22,150,175,0.6)\"><\/i>\u03c0 molecular orbital (replaces two p orbitals)<\/span>\n    <span><i class=\"k-ball\" style=\"background:rgba(214,150,24,0.9)\"><\/i>Higher-priority group (CH\u2083)<\/span>\n  <\/div>\n<\/div>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/ez-rotation.js?v=20261019d\"><\/script>\n<\/section>\n\n<p class=\"ols-cc-ezrt-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n\n      <article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Before You Start<\/h2>\n<\/div>\n<p>This page builds on GCSE work on isomers, displayed formulae and the C=C bond in alkenes.<\/p>\n<ul class=\"ols-list\">\n<li><strong>Isomers<\/strong> have the same molecular formula but a different arrangement of atoms.<\/li>\n<li>Alkenes contain a C=C double bond and have the general formula C<sub>n<\/sub>H<sub>2n<\/sub>.<\/li>\n<li>A displayed formula shows every atom and every bond in a molecule.<\/li>\n<li>The <strong>atomic number<\/strong> of an element is the number of protons in the nucleus of its atom.<\/li>\n<\/ul>\n<\/article>\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: Isomers and Atomic Number<\/h2>\n<p>Four quick GCSE questions on isomers, alkene formulae and atomic number before you meet stereoisomers.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1267\"><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">2<\/div><h2>What Are Stereoisomers?<\/h2><\/div>\n        <p><strong>Stereoisomers<\/strong> are compounds with the same structural formula but a different three-dimensional arrangement of atoms in space.<\/p>\n        <p>In alkenes, this matters because the <strong>C=C double bond restricts rotation<\/strong>. The atoms or groups attached to the double bond cannot freely rotate into a new arrangement without breaking the pi bond.<\/p>\n        <p>This produces a type of stereoisomerism called <strong>E-Z stereoisomerism<\/strong>, also called geometric isomerism.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Key idea:<\/strong> E-Z isomers are not structural isomers. They have the same connectivity, but different spatial arrangement around the carbon-carbon double bond.<\/p><\/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: Structural Isomer or Stereoisomer?<\/h2>\n<p>Complete a comparison of three alkenes that are not on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-17\" class=\"h5p-iframe\" data-content-id=\"17\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Geometric Isomerism Drag: Comparing the Pentenes\"><\/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>Why E-Z Isomerism Occurs<\/h2><\/div>\n        <p>E-Z isomerism occurs because the carbon-carbon double bond contains a <strong>pi bond<\/strong>. Rotation around the C=C bond is restricted because rotating one carbon would break the sideways overlap of the p orbitals.<\/p>\n        <p>By contrast, a carbon-carbon single bond can rotate freely. This is why simple alkanes do not usually produce this type of fixed geometric arrangement.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Exam focus:<\/strong> Always link E-Z isomerism to <strong>restricted rotation around the C=C double bond<\/strong>.<\/p><\/div>\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\">\n            <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t4-mv-12-displayed.svg\" alt=\"Displayed formulae of (Z)-but-2-ene, (E)-but-2-ene\" width=\"1514\" height=\"1071\">\n          <\/div>\n        <\/div>\n        \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 Two Forms Against One<\/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-18\" class=\"h5p-iframe\" data-content-id=\"18\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Geometric Isomerism Explain: 1,2-Dibromoethene and 1,2-Dibromoethane\"><\/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>Conditions for E-Z Stereoisomerism<\/h2><\/div>\n        <p>For an alkene to show E-Z stereoisomerism, <strong>two conditions<\/strong> must both be met:<\/p>\n        <ol>\n          <li>There must be <strong>restricted rotation<\/strong> about the carbon-carbon double bond.<\/li>\n          <li>Each carbon atom of the C=C bond must be attached to <strong>two different atoms or groups<\/strong>.<\/li>\n        <\/ol>\n        <p>If either carbon in the C=C bond has two identical groups attached, E-Z isomerism is not possible.<\/p>\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\">\n            <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t4-cd-15-butene1noez.svg\" alt=\"But-1-ene with the two hydrogen atoms on carbon 1 ringed: identical groups, so no E\/Z isomers\" width=\"1518\" height=\"946\">\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-18-ezflow.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-18-ezflow.jpg\" alt=\"Flowchart: is there a C=C, does each carbon carry two different groups; yes to all gives E\/Z isomers, with a locked hinge for the \u03c0 bond\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-18-ezflow.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Two checks decide E\/Z isomerism: restricted rotation about C=C, and two different groups on each carbon of the double bond.<\/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: Which Alkenes Show E-Z Isomerism?<\/h2>\n<p>Sketch each alkene, then click every one that has E and Z isomers.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-297\" class=\"h5p-iframe\" data-content-id=\"297\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Geometric Isomerism Mark the Words: Which Alkenes Show E-Z Isomerism\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">5<\/div><h2>Assigning E and Z<\/h2><\/div>\n        <p>To name an alkene with E-Z stereoisomerism, first identify the <strong>priority group<\/strong> on each carbon atom of the double bond.<\/p>\n        <p>The priority group is found by comparing the atoms directly attached to each double-bond carbon. The atom with the <strong>higher atomic number<\/strong> has higher priority.<\/p>\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Arrangement of priority groups<\/th>\n                <th>Name used<\/th>\n                <th>Memory aid<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td>Priority groups on the same side of the C=C bond<\/td>\n                <td><strong>Z<\/strong><\/td>\n                <td><strong>Z<\/strong> comes from <em>zusammen<\/em>, meaning together.<\/td>\n              <\/tr>\n              <tr>\n                <td>Priority groups on opposite sides of the C=C bond<\/td>\n                <td><strong>E<\/strong><\/td>\n                <td><strong>E<\/strong> comes from <em>entgegen<\/em>, meaning opposite.<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\">\n            <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t4-cd-16-dichloroethene.svg\" alt=\"(Z)- and (E)-1,2-dichloroethene with Cl marked as the priority group on each carbon\" width=\"1520\" height=\"1054\">\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: E, Z or Neither?<\/h2>\n<p>Find the priority group on each carbon atom, then decide quickly.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-20\" class=\"h5p-iframe\" data-content-id=\"20\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Geometric Isomerism Quick-Fire: E, Z or Neither\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">6<\/div><h2>Cis-Trans Isomerism<\/h2><\/div>\n        <p><strong>Cis-trans isomerism<\/strong> is a simpler special case of E-Z isomerism. It can be used when two of the substituent groups attached to the double bond are identical.<\/p>\n        <p>For but-2-ene, the two CH<sub>3<\/sub> groups can be on the same side or on opposite sides of the double bond.<\/p>\n        <div class=\"ols-comparison-grid\">\n          <div class=\"ols-mini-card\">\n            <div class=\"ols-mini-card-image\">\n              <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/skel-P008-z-but-2-ene.svg\" alt=\"Displayed formula of (Z)-but-2-ene, with both CH3 groups on the same side of the C=C double bond\">\n            <\/div>\n            <div class=\"ols-mini-card-body\">\n              <h3>Z-but-2-ene<\/h3>\n              <p>The two CH<sub>3<\/sub> groups are on the same side, so this can also be called <strong>cis-but-2-ene<\/strong>.<\/p>\n            <\/div>\n          <\/div>\n          <div class=\"ols-mini-card\">\n            <div class=\"ols-mini-card-image\">\n              <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t4-cd-14-zebutene.svg\" alt=\"(Z)- and (E)-but-2-ene side by side with the CH\u2083 groups ringed and the C=C axis dashed\">\n            <\/div>\n            <div class=\"ols-mini-card-body\">\n              <h3>E-but-2-ene<\/h3>\n              <p>The two CH<sub>3<\/sub> groups are on opposite sides, so this can also be called <strong>trans-but-2-ene<\/strong>.<\/p>\n            <\/div>\n          <\/div>\n        <\/div>\n        <div class=\"ols-key-box\"><p><strong>Important:<\/strong> E-Z notation is the more general and accurate naming system, especially when the groups attached to the C=C bond are not identical.<\/p><\/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: Pick the Accurate Statement<\/h2>\n<p>Each round puts a common mistake next to the correct idea, in a molecule you have not met on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-19\" class=\"h5p-iframe\" data-content-id=\"19\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Geometric Isomerism Summary: Misconceptions in New Molecules\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">7<\/div><h2>Common Exam Mistakes<\/h2><\/div>\n        <ul>\n          <li>Do not call E-Z isomers <strong>structural isomers<\/strong>. They have the same structural formula.<\/li>\n          <li>Do not assign E or Z by only looking at the largest group visually. Use <strong>atomic number priority<\/strong>.<\/li>\n          <li>Do not assume every alkene shows E-Z isomerism. Each carbon in the C=C bond must have two different groups attached.<\/li>\n          <li>Do not say the molecule rotates around the double bond. The whole point is that rotation around C=C is restricted.<\/li>\n        <\/ul>\n      <div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-19-eznaming.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-19-eznaming.jpg\" alt=\"Coloured-blob diagrams for Z (together) and E (opposite), when cis\/trans applies, and three wrong-versus-right exam statements\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-19-eznaming.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Z means the priority groups are on the same side and E on opposite sides; cis and trans are used only when the groups match.<\/p><\/div>\n<\/div>\n<\/article>\n\n      \n\n      <section class=\"ols-infographic-full\" aria-label=\"Geometric isomerism infographic summary\">\n        <div class=\"ols-infographic-full-frame\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/skel-02-geometric-isomerism-a.svg\" alt=\"Geometric isomerism: definition, conditions, and the Z and E isomers of but-2-ene\" width=\"1410\" height=\"963\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/skel-02-geometric-isomerism-b.svg\" alt=\"CIP priority rule applied to Z- and E-1,2-dichloroethene, and but-1-ene which has no E\/Z isomers\" width=\"1501\" height=\"1175\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/skel-02-geometric-isomerism-c.svg\" alt=\"How cis and trans relate to Z and E, with a short exam recap\" width=\"1502\" height=\"1011\">\n        <\/div>\n      <\/section>\n\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>These questions secure the decisions that cost marks when you identify and name E-Z isomers.<\/p>\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Does but-1-ene have E and Z isomers?<\/h3>\n<p>No. Carbon 1 of the C=C bond carries two hydrogen atoms, so swapping the groups on that carbon gives the same molecule. E-Z isomerism needs two different groups on each carbon of the double bond.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Is the Z isomer always the cis isomer?<\/h3>\n<p>Not always. Cis and trans compare identical groups, while E and Z compare priority groups. In 2-bromobut-2-ene, the isomer with both CH<sub>3<\/sub> groups on the same side is cis, but it is named E because Br outranks CH<sub>3<\/sub>.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Do I decide priority by the size of a group or by atomic number?<\/h3>\n<p>By the atomic number of the atom directly attached to the double-bond carbon. Size alone does not count: Cl outranks a CH<sub>2<\/sub>CH<sub>3<\/sub> group because chlorine has atomic number 17 and carbon has 6.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Do I compare groups on the same carbon or across the double bond?<\/h3>\n<p>Both, in that order. First choose the higher-priority group on each carbon separately. Then compare where those two priority groups sit: on the same side is Z, on opposite sides is E.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Are the E and Z isomers really different compounds?<\/h3>\n<p>Yes. They cannot turn into each other at room temperature because that would mean breaking the \u03c0 bond. They can be separated and have different physical properties, such as their boiling temperatures.<\/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-related-card\">\n        <h2>Related Alkene Revision Notes<\/h2>\n        <p>Continue through Topic 5 by linking geometric isomerism to alkene bonding, electrophilic addition and polymerisation.<\/p>\n        <div class=\"ols-related-grid\">\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/alkene-bonding\/\">Alkene Bonding<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/electrophilic-addition-mechanism\/\">Electrophilic Addition Mechanism<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Reactions of Alkenes<\/a>\n        <\/div>\n      <\/section>\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<\/section>\n\n<script type=\"application\/ld+json\">\n{\n    \"@context\": \"https:\/\/schema.org\",\n    \"@graph\": [\n        {\n            \"@type\": \"WebPage\",\n            \"@id\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/geometric-isomerism\/#webpage\",\n            \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/geometric-isomerism\/\",\n            \"name\": \"Geometric Isomerism\",\n            \"headline\": \"Geometric Isomerism\",\n            \"description\": \"A concise revision guide to stereoisomerism in alkenes, restricted rotation around C=C bonds, E-Z naming, priority groups and the link between E-Z and cis-trans isomerism.\",\n            \"inLanguage\": \"en-GB\",\n            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\"acceptedAnswer\": {\n                        \"@type\": \"Answer\",\n                        \"text\": \"No. Carbon 1 of the C=C bond carries two hydrogen atoms, so swapping the groups on that carbon gives the same molecule. E-Z isomerism needs two different groups on each carbon of the double bond.\"\n                    }\n                },\n                {\n                    \"@type\": \"Question\",\n                    \"name\": \"Is the Z isomer always the cis isomer?\",\n                    \"acceptedAnswer\": {\n                        \"@type\": \"Answer\",\n                        \"text\": \"Not always. Cis and trans compare identical groups, while E and Z compare priority groups. In 2-bromobut-2-ene, the isomer with both CH3 groups on the same side is cis, but it is named E because Br outranks CH3.\"\n                    }\n                },\n                {\n                    \"@type\": \"Question\",\n                    \"name\": \"Do I decide priority by the size of a group or by atomic number?\",\n                    \"acceptedAnswer\": {\n                        \"@type\": \"Answer\",\n                        \"text\": \"By the atomic number of the atom directly attached to the double-bond carbon. Size alone does not count: Cl outranks a CH2CH3 group because chlorine has atomic number 17 and carbon has 6.\"\n                    }\n                },\n                {\n                    \"@type\": \"Question\",\n                    \"name\": \"Do I compare groups on the same carbon or across the double bond?\",\n                    \"acceptedAnswer\": {\n                        \"@type\": \"Answer\",\n                        \"text\": \"Both, in that order. First choose the higher-priority group on each carbon separately. Then compare where those two priority groups sit: on the same side is Z, on opposite sides is E.\"\n                    }\n                },\n                {\n                    \"@type\": \"Question\",\n                    \"name\": \"Are the E and Z isomers really different compounds?\",\n                    \"acceptedAnswer\": {\n                        \"@type\": \"Answer\",\n                        \"text\": \"Yes. They cannot turn into each other at room temperature because that would mean breaking the \u03c0 bond. They can be separated and have different physical properties, such as their boiling temperatures.\"\n                    }\n                }\n            ]\n        }\n    ]\n}\n<\/script>\n\n\n\n<p class=\"wp-block-paragraph\"><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Revision Notes \/ A Level Chemistry \/ Edexcel International \/ Topic 5 Alkenes \/ Geometric Isomerism Geometric Isomerism A concise revision guide to stereoisomerism in alkenes, restricted rotation around C=C bonds, E-Z naming, priority groups and the link between E-Z and cis-trans isomerism. 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