{"id":8100,"date":"2026-09-15T08:35:09","date_gmt":"2026-09-15T07:35:09","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/ions-and-ionic-formulae\/"},"modified":"2026-09-22T10:07:49","modified_gmt":"2026-09-22T09:07:49","slug":"ions-and-ionic-formulae","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/ions-and-ionic-formulae\/","title":{"rendered":"Ions and Ionic Formulae"},"content":{"rendered":"\n<!--\n===============================================================================\nONLINE LEARNING SYSTEM COPYRIGHT NOTICE\n\u00a9 Online Learning System. 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background: #2563eb !important;\n      color: #ffffff !important;\n    }\n\n    body.ols-lightbox-lock {\n      overflow: hidden !important;\n    }\n\n    @media (max-width: 760px) {\n      .ols-zoom-card {\n        border-radius: 22px !important;\n        overflow: hidden !important;\n      }\n\n      .ols-zoom-card-image {\n        min-height: auto !important;\n        padding: 12px !important;\n        overflow: hidden !important;\n        border-top-left-radius: 22px !important;\n        border-top-right-radius: 22px !important;\n      }\n\n      .ols-zoom-card img.ols-zoomable-img,\n      .ols-zoom-card img.ols-zoomable-img:hover {\n        transform: none !important;\n        box-shadow: none !important;\n      }\n\n      .ols-zoom-card-caption {\n        border-bottom-left-radius: 22px !important;\n        border-bottom-right-radius: 22px !important;\n      }\n\n      .ols-image-lightbox {\n        padding: 14px !important;\n      }\n\n      .ols-image-lightbox-img {\n        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<p>Cambridge International AS Level Chemistry<\/p>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>Topic 2 Atoms, Molecules and Stoichiometry<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/\">Topic overview<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/atoms-elements-and-molecules\/\">2.1 \/ 2.2 Atoms, Elements and Molecules<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/ions-and-ionic-formulae\/\">2.3 Ions and Ionic Formulae<\/a><\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/\">2.3 Equations and Reaction Types<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/writing-chemical-equations\/\">Writing Chemical Equations<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/typical-reactions-of-acids\/\">Typical Reactions of Acids<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/ionic-and-full-equations\/\">Ionic and Full Equations<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/empirical-and-molecular-formulae\/\">2.3 Empirical and Molecular Formulae<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/empirical-and-molecular-formulae\/empirical-formulae\/\">Empirical Formulae<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/empirical-and-molecular-formulae\/molecular-formulae\/\">Molecular Formulae<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/hydrated-salts-and-water-of-crystallisation\/\">2.3 Hydrated Salts and Water of Crystallisation<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/\">2.4 Mole Calculations<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/comparing-masses-of-substances\/\">Comparing Masses of Substances<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/calculations-involving-moles\/\">Calculations involving Moles<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/calculations-using-reacting-masses\/\">Calculations using Reacting Masses<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/the-yield-of-a-reaction\/\">The Yield of a Reaction<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/atom-economy\/\">Atom Economy<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/calculations-with-solutions-and-gases\/\">2.4 Solutions and Gases<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/calculations-with-solutions-and-gases\/molar-volume-calculations\/\">Molar Volume Calculations<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/calculations-with-solutions-and-gases\/concentrations-of-solutions\/\">Concentrations of Solutions<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/calculations-with-solutions-and-gases\/titration-calculations\/\">Titration Calculations<\/a><\/li>\n        <\/ul>\n      <\/li>\n    <\/ul>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>Other Topics<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-1-atomic-structure\/\">Topic 1 Atomic Structure<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/\">Topic 3 Chemical Bonding<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-4-states-of-matter\/\">Topic 4 States of Matter<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-9-the-periodic-table-chemical-periodicity\/\">Topic 9 Chemical Periodicity<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-13-an-introduction-to-as-level-organic-chemistry\/\">Topic 13 Introduction to Organic Chemistry<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-14-hydrocarbons\/\">Topic 14 Hydrocarbons<\/a><\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n    return String(path || '')\r\n      .split('?')[0]\r\n      .split('#')[0]\r\n      .replace(\/\\\/+$\/, '')\r\n      .toLowerCase();\r\n  }\r\n\r\n  function highlightActive() {\r\n    var sidebar = document.querySelector('.ols-sidebar');\r\n    if (!sidebar) return false;\r\n\r\n    var currentPath = normalisePath(window.location.pathname);\r\n    var links = sidebar.querySelectorAll('a[href]');\r\n    var matched = null;\r\n    var matchedLength = 0;\r\n\r\n    sidebar.querySelectorAll('.active, .active-main, .parent-active').forEach(function(item) {\r\n      item.classList.remove('active', 'active-main', 'parent-active');\r\n    });\r\n\r\n    sidebar.querySelectorAll('a[data-ols-disabled-parent=\"1\"], 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<h1>Ions and Ionic Formulae<\/h1>\n        <p class=\"ols-page-intro\">A concise Cambridge International AS Level Chemistry revision guide to predicting the charge on a simple ion from its group, the nine named ions including zinc, silver, hydrogencarbonate and phosphate, formulae from oxidation numbers shown as Roman numerals, and constructing the formula of any ionic compound (2.3.1).<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">AS Level<\/div>\n<div class=\"ols-badge\">Topic 2: Atoms, Molecules and Stoichiometry<\/div>\n<div class=\"ols-badge\">9701 Papers 1 and 2<\/div>\n<\/div>\n\n        <div class=\"ols-author\">\n          <img decoding=\"async\" class=\"ols-author-avatar-img\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\" alt=\"Dr. Mohammed Al-Fatah\">\n          <div class=\"ols-author-content\">\n            <h2 class=\"ols-author-title\">Written by:<br><span>Dr. Mohammed Al-Fatah<\/span><\/h2>\n            <p class=\"ols-author-description\">Chemistry specialist revision notes for A Level Chemistry.<\/p>\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\"><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\"><\/path><\/svg>\n              View LinkedIn Profile\n            <\/a>\n          <\/div>\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: Losing and Gaining Electrons<\/h2>\n<p>Before you start, check the GCSE picture of how ions are formed.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"807\"><\/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>Predicting the Charge on a Simple Ion<\/h2>\n<\/div>\n<p>A metal atom in Groups 1, 2 or 13 loses its outer electrons to reach the electron configuration of the previous noble gas; a non-metal atom in Groups 15, 16 or 17 gains electrons to reach the next. The number of electrons lost or gained is the <strong>charge on the ion<\/strong>, and it can be read straight from the group number.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Group<\/th><th>Electrons lost or gained<\/th><th>Ion charge<\/th><th>Examples<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>1<\/strong><\/td><td>Lose 1<\/td><td>1+<\/td><td>Li<sup>+<\/sup>, Na<sup>+<\/sup>, K<sup>+<\/sup><\/td><\/tr>\n<tr><td><strong>2<\/strong><\/td><td>Lose 2<\/td><td>2+<\/td><td>Mg<sup>2+<\/sup>, Ca<sup>2+<\/sup>, Ba<sup>2+<\/sup><\/td><\/tr>\n<tr><td><strong>3<\/strong><\/td><td>Lose 3<\/td><td>3+<\/td><td>Al<sup>3+<\/sup><\/td><\/tr>\n<tr><td><strong>5<\/strong><\/td><td>Gain 3<\/td><td>3-<\/td><td>N<sup>3-<\/sup>, P<sup>3-<\/sup><\/td><\/tr>\n<tr><td><strong>6<\/strong><\/td><td>Gain 2<\/td><td>2-<\/td><td>O<sup>2-<\/sup>, S<sup>2-<\/sup><\/td><\/tr>\n<tr><td><strong>7<\/strong><\/td><td>Gain 1<\/td><td>1-<\/td><td>F<sup>&#8211;<\/sup>, Cl<sup>&#8211;<\/sup>, Br<sup>&#8211;<\/sup>, I<sup>&#8211;<\/sup><\/td><\/tr>\n<tr><td><strong>Transition metals<\/strong><\/td><td>Variable<\/td><td>Given in the name<\/td><td>Fe<sup>2+<\/sup> in iron(II), Fe<sup>3+<\/sup> in iron(III)<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Metals form positive ions with the charge equal to the group number; non-metals form negative ions with the charge equal to 8 minus the group number.<\/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: Charges from a Neighbour<\/h2>\n<p>Five rapid questions; each one names an element in the same group to work from.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-469\" class=\"h5p-iframe\" data-content-id=\"469\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ionic Formulae Quick Fire: Charges from a Neighbour in the Same Group\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>The Compound Ions You Must Know<\/h2>\n<\/div>\n<p>Some ions are groups of atoms covalently bonded together that carry an overall charge. Cambridge names five that you must be able to write from memory.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Ion<\/th><th>Formula<\/th><th>Charge<\/th><th>Typical compound<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Sulfate<\/strong><\/td><td>SO<sub>4<\/sub><sup>2-<\/sup><\/td><td>2-<\/td><td>MgSO<sub>4<\/sub><\/td><\/tr>\n<tr><td><strong>Hydroxide<\/strong><\/td><td>OH<sup>&#8211;<\/sup><\/td><td>1-<\/td><td>Ca(OH)<sub>2<\/sub><\/td><\/tr>\n<tr><td><strong>Nitrate<\/strong><\/td><td>NO<sub>3<\/sub><sup>&#8211;<\/sup><\/td><td>1-<\/td><td>KNO<sub>3<\/sub><\/td><\/tr>\n<tr><td><strong>Carbonate<\/strong><\/td><td>CO<sub>3<\/sub><sup>2-<\/sup><\/td><td>2-<\/td><td>Na<sub>2<\/sub>CO<sub>3<\/sub><\/td><\/tr>\n<tr><td><strong>Ammonium<\/strong><\/td><td>NH<sub>4<\/sub><sup>+<\/sup><\/td><td>1+<\/td><td>NH<sub>4<\/sub>Cl<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/common-compound-ions-structures-and-charges.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\/common-compound-ions-structures-and-charges.jpg\" alt=\"Reference card of the common compound ions: sulfate, hydroxide, nitrate, carbonate and ammonium, each with its structure and charge\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The five compound ions Cambridge expects you to recall.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Ammonium is the only common positive compound ion. Sulfate and carbonate are 2-; hydroxide and nitrate are 1-.<\/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: Inside a Compound Ion<\/h2>\n<p>Decide whether the statement about ammonium nitrate is true or false.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-808\" class=\"h5p-iframe\" data-content-id=\"808\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ionic Formulae True or False: Two Nitrogens in Ammonium Nitrate\"><\/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>Constructing the Formula of an Ionic Compound<\/h2>\n<\/div>\n<p>An ionic compound is neutral overall, so the positive and negative charges must cancel. Use the <strong>charge-balance method<\/strong>: write the ions, find the lowest number of each that makes the total charge zero, and write the formula with those numbers as subscripts. Brackets go round a compound ion when more than one is needed.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Compound<\/th><th>Ions<\/th><th>Balance the charges<\/th><th>Formula<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Magnesium chloride<\/strong><\/td><td>Mg<sup>2+<\/sup>, Cl<sup>&#8211;<\/sup><\/td><td>One 2+ needs two 1-<\/td><td>MgCl<sub>2<\/sub><\/td><\/tr>\n<tr><td><strong>Aluminium oxide<\/strong><\/td><td>Al<sup>3+<\/sup>, O<sup>2-<\/sup><\/td><td>Two 3+ (6+) balance three 2- (6-)<\/td><td>Al<sub>2<\/sub>O<sub>3<\/sub><\/td><\/tr>\n<tr><td><strong>Calcium nitrate<\/strong><\/td><td>Ca<sup>2+<\/sup>, NO<sub>3<\/sub><sup>&#8211;<\/sup><\/td><td>One 2+ needs two nitrates; bracket the compound ion<\/td><td>Ca(NO<sub>3<\/sub>)<sub>2<\/sub><\/td><\/tr>\n<tr><td><strong>Ammonium sulfate<\/strong><\/td><td>NH<sub>4<\/sub><sup>+<\/sup>, SO<sub>4<\/sub><sup>2-<\/sup><\/td><td>Two 1+ balance one 2-<\/td><td>(NH<sub>4<\/sub>)<sub>2<\/sub>SO<sub>4<\/sub><\/td><\/tr>\n<tr><td><strong>Iron(III) hydroxide<\/strong><\/td><td>Fe<sup>3+<\/sup>, OH<sup>&#8211;<\/sup><\/td><td>One 3+ needs three 1-<\/td><td>Fe(OH)<sub>3<\/sub><\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>A quick check: the total positive charge equals the total negative charge, and the subscripts are the smallest whole numbers that achieve that. The &#8220;swap the charges&#8221; shortcut gives the same answer but simplify afterwards: Mg<sup>2+<\/sup> with O<sup>2-<\/sup> is MgO, not Mg<sub>2<\/sub>O<sub>2<\/sub>.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> The formula of an ionic compound shows the simplest whole-number ratio of ions that gives no overall charge.<\/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: Build Four Formulae<\/h2>\n<p>Drag the correct formula into each blank; the ion charges are given for you.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-468\" class=\"h5p-iframe\" data-content-id=\"468\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ionic Formulae Drag: Building Formulae from Ion Charges\"><\/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: Which Formulae Are Wrong?<\/h2>\n<p>Click every formula in the list that has been written incorrectly.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-470\" class=\"h5p-iframe\" data-content-id=\"470\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ionic Formulae Mark the Words: Spot the Formulae That Are Wrong\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Common Exam Mistakes<\/h2>\n<\/div>\n<ul>\n<li>Writing NaCl<sub>2<\/sub> or MgCl. Check that the charges cancel: Na<sup>+<\/sup> needs one Cl<sup>&#8211;<\/sup>, Mg<sup>2+<\/sup> needs two.<\/li>\n<li>Leaving out brackets: CaNO<sub>32<\/sub> instead of Ca(NO<sub>3<\/sub>)<sub>2<\/sub>.<\/li>\n<li>Giving sulfate as SO<sub>4<\/sub><sup>&#8211;<\/sup> or carbonate as CO<sub>3<\/sub><sup>&#8211;<\/sup>. Both carry a 2- charge.<\/li>\n<li>Writing a molecular formula for a giant lattice. NaCl is a ratio of ions, not a molecule of one sodium and one chlorine.<\/li>\n<li>Forgetting the Roman numeral: iron chloride is ambiguous; FeCl<sub>2<\/sub> is iron(II) chloride and FeCl<sub>3<\/sub> is iron(III) chloride.<\/li>\n<\/ul>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> Charges cancel, smallest ratio, brackets round repeated compound ions.<\/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: Read a Formula Backwards<\/h2>\n<p>Work out the charge on the metal ion from the formula you are given.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-471\" class=\"h5p-iframe\" data-content-id=\"471\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ionic Formulae MCQ: Reading a Formula Backwards\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card orange\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>The Nine Named Ions and Roman-Numeral Formulae<\/h2>\n<\/div>\n<p>Cambridge adds four ions to the five above and asks for formulae written from oxidation numbers.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Ion<\/th><th>Formula<\/th><th>Example compound<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Zinc<\/strong><\/td><td>Zn\u00b2\u207a<\/td><td>ZnSO\u2084<\/td><\/tr>\n<tr><td><strong>Silver<\/strong><\/td><td>Ag\u207a<\/td><td>AgNO\u2083<\/td><\/tr>\n<tr><td><strong>Hydrogencarbonate<\/strong><\/td><td>HCO\u2083\u207b<\/td><td>NaHCO\u2083, Ca(HCO\u2083)\u2082<\/td><\/tr>\n<tr><td><strong>Phosphate<\/strong><\/td><td>PO\u2084\u00b3\u207b<\/td><td>Na\u2083PO\u2084, Ca\u2083(PO\u2084)\u2082<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p><strong>Roman numerals give the charge.<\/strong> In a name such as iron(III) chloride or copper(II) sulfate, the numeral is the oxidation number of the metal, which for a simple ion equals its charge. Iron(III) is Fe\u00b3\u207a, so iron(III) chloride is FeCl\u2083; iron(II) is Fe\u00b2\u207a, so iron(II) chloride is FeCl\u2082. Copper(II) sulfate is CuSO\u2084; manganese(IV) oxide is MnO\u2082 because Mn\u2074\u207a balances two O\u00b2\u207b.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Name<\/th><th>Metal ion<\/th><th>Anion<\/th><th>Formula<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Iron(III) sulfate<\/strong><\/td><td>Fe\u00b3\u207a<\/td><td>SO\u2084\u00b2\u207b<\/td><td>Fe\u2082(SO\u2084)\u2083<\/td><\/tr>\n<tr><td><strong>Copper(I) oxide<\/strong><\/td><td>Cu\u207a<\/td><td>O\u00b2\u207b<\/td><td>Cu\u2082O<\/td><\/tr>\n<tr><td><strong>Lead(II) nitrate<\/strong><\/td><td>Pb\u00b2\u207a<\/td><td>NO\u2083\u207b<\/td><td>Pb(NO\u2083)\u2082<\/td><\/tr>\n<tr><td><strong>Chromium(III) hydroxide<\/strong><\/td><td>Cr\u00b3\u207a<\/td><td>OH\u207b<\/td><td>Cr(OH)\u2083<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> The Roman numeral in a name is the oxidation number of the metal, which gives the charge on its ion; balance that charge against the anion to write the formula.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: The Extra Ions and Roman Numerals<\/h2>\n<p>Four rapid questions on the ions added by this card.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-519\" class=\"h5p-iframe\" data-content-id=\"519\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ionic Formulae Quick Fire: Hydrogencarbonate, Phosphate and Roman Numerals\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card purple\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Where This Sits in the Cambridge Syllabus<\/h2>\n<\/div>\n<p>Cambridge 2.3.1 asks you to write the formulae of ionic compounds from ionic charges and from oxidation numbers shown as Roman numerals, and to recall the nitrate, carbonate, sulfate, hydroxide, ammonium, zinc, silver, hydrogencarbonate and phosphate ions. This page covers the charges, the nine ions and the balancing method.<\/p>\n<\/article>\n\n<section class=\"ols-related-card\">\n        <h2>Related Topics<\/h2>\n        <p>Continue through Topic 2 by linking this page to equations, formulae and mole calculation revision.<\/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-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/\">Equations and Reaction Types<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/writing-chemical-equations\/\">Writing Chemical Equations<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/\">Mole Calculations<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/empirical-and-molecular-formulae\/\">Empirical and Molecular Formulae<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/calculations-with-solutions-and-gases\/\">Calculations with Solutions and Gases<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-1-atomic-structure\/\">Atomic Structure &amp; The Periodic Table<\/a>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-attribution-card\">\n        <p><strong>Copyright notice:<\/strong> This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.<\/p>\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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/ions-and-ionic-formulae\/#webpage\",\n      \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/ions-and-ionic-formulae\/\",\n      \"name\": \"Ions and Ionic Formulae - Cambridge International AS Level Chemistry\",\n      \"description\": \"Cambridge International AS Level Chemistry 2.3.1 revision notes: ion charges from the Periodic Table, the nine named ions, Roman-numeral oxidation numbers and constructing ionic formulae.\",\n      \"inLanguage\": \"en-GB\",\n      \"isPartOf\": {\n        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