{"id":8051,"date":"2026-09-15T08:23:34","date_gmt":"2026-09-15T07:23:34","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/atoms-elements-and-molecules\/"},"modified":"2026-09-24T11:05:34","modified_gmt":"2026-09-24T10:05:34","slug":"atoms-elements-and-molecules","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/atoms-elements-and-molecules\/","title":{"rendered":"Atoms, Elements and Molecules"},"content":{"rendered":"\n<!--\n===============================================================================\nONLINE LEARNING SYSTEM COPYRIGHT NOTICE\n\u00a9 Online Learning System. 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padding: 8px 10px; font-size: 11px; font-weight: 700; color: #7a4b12; line-height: 1.45; text-align: center; }\n<\/style>\n\n  <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>OCR A Level Chemistry A<\/p>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>2.1.3 Amount of Substance<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/\">Section overview<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/atoms-elements-and-molecules\/\">Atoms, Elements and Molecules<\/a><\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/\">Mole Calculations<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/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\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/calculations-involving-moles\/\">Calculations involving Moles<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/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\/ocr-a\/2-1-3-amount-of-substance\/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\/ocr-a\/2-1-3-amount-of-substance\/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\/ocr-a\/2-1-3-amount-of-substance\/empirical-and-molecular-formulae\/\">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\/ocr-a\/2-1-3-amount-of-substance\/empirical-and-molecular-formulae\/empirical-formulae\/\">Empirical Formulae<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/empirical-and-molecular-formulae\/molecular-formulae\/\">Molecular Formulae<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/calculations-with-solutions-and-gases\/\">Calculations with Solutions and Gases<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/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\/ocr-a\/2-1-3-amount-of-substance\/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\/ocr-a\/2-1-3-amount-of-substance\/hydrated-salts-and-water-of-crystallisation\/\">Hydrated Salts and Water of Crystallisation<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/measuring-mass-volume-and-gas-volume\/\">Measuring Mass, Volume and Gas Volume<\/a><\/li>\n        <\/ul>\n      <\/li>\n    <\/ul>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>Other OCR Sections<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-1-atomic-structure-and-isotopes\/\">2.1.1 Atomic Structure and Isotopes<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-2-compounds-formulae-and-equations\/\">2.1.2 Compounds, Formulae and Equations<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-4-acids\/\">2.1.4 Acids<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-1-electron-structure\/\">2.2.1 Electron Structure<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/\">2.2.2 Bonding and Structure<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-1-1-periodicity\/\">3.1.1 Periodicity<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/4-1-1-basic-concepts-of-organic-chemistry\/\">4.1.1 Basic Concepts of Organic Chemistry<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/4-1-2-alkanes\/\">4.1.2 Alkanes<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/4-1-3-alkenes\/\">4.1.3 Alkenes<\/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\"], a[aria-current]').forEach(function(a) {\r\n      a.removeAttribute('aria-current');\r\n      a.removeAttribute('tabindex');\r\n      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{\r\n    document.addEventListener('DOMContentLoaded', highlightActive);\r\n  }\r\n})();\r\n<\/script>\n\n  <main class=\"ols-main\">\n      <nav class=\"ols-breadcrumbs\" aria-label=\"Breadcrumb\">\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/\">Revision Notes<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/\">A Level Chemistry<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/\">OCR A<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/\">2.1.3 Amount of Substance<\/a> \/\n<span>Atoms, Elements and Molecules<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Atoms, Elements and Molecules<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to the meaning of atoms, elements, molecules, compounds and ions, including ionic charges, simple ionic formulae and monatomic, diatomic and polyatomic terminology.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Paper 1 and 2<\/div>\n<div class=\"ols-badge\">2.1.3 Amount of Substance<\/div>\n<div class=\"ols-badge\">H432\/01 and H432\/02<\/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      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">1<\/div><h2>What Is an Element?<\/h2><\/div>\n        <p>An <strong>element<\/strong> is a substance made from only one type of atom. Each element is represented by a chemical symbol, such as <strong>H<\/strong> for hydrogen, <strong>O<\/strong> for oxygen and <strong>Na<\/strong> for sodium.<\/p>\n        <p>Some element symbols have one capital letter. Others have two letters, where the first letter is capitalised and the second is lower case. For example, <strong>Cl<\/strong> represents chlorine, not C and l separately.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Key idea:<\/strong> an element cannot be broken down into simpler substances by chemical reactions.<\/p><\/div>\n      <\/article>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">2<\/div><h2>What Is an Atom?<\/h2><\/div>\n        <p>An <strong>atom<\/strong> is the smallest particle of an element that has the chemical properties of that element. For example, one oxygen atom still behaves as oxygen.<\/p>\n        <p>At this stage, you should remember that atoms contain smaller particles: <strong>protons<\/strong>, <strong>neutrons<\/strong> and <strong>electrons<\/strong>. You will study these in more detail in this section: Atomic Structure and the Periodic Table.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Definition:<\/strong> an atom is the smallest particle of an element that can take part in chemical change while retaining the identity of that element.<\/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: Symbols and Formulae<\/h2>\n<p>Decide what a pair of capital letters really says about a compound.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-845\" class=\"h5p-iframe\" data-content-id=\"845\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Atoms and Elements MCQ: Cobalt, Carbon and a Capital Letter\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card purple ols-zoom-parent\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">3<\/div><h2>What Is a Molecule?<\/h2><\/div>\n        <p>A <strong>molecule<\/strong> is a particle made from two or more atoms chemically bonded together.<\/p>\n        <p>If the atoms are all from the same element, the particle is a <strong>molecule of an element<\/strong>. For example, H<sub>2<\/sub> is a molecule of hydrogen because it contains two hydrogen atoms bonded together.<\/p>\n        <p>If the atoms are from different elements, the particle is a <strong>molecule of a compound<\/strong>. For example, H<sub>2<\/sub>O contains hydrogen and oxygen atoms bonded together.<\/p>\n\n        <div class=\"ols-zoom-card\">\n          <div class=\"ols-figure-card\">\n<div class=\"ols-figure-image\"><img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/fix-62.jpg\" alt=\"Reference table of the particle terms element, atom, molecule, compound and ion, each with a diagram, name and formula\"><\/div>\n<div class=\"ols-figure-caption\"><p>Educational chart showing atoms, elements, molecules, compounds and ions<\/p><\/div>\n<\/div>\n          <div class=\"ols-zoom-card-caption\">\n            <p>This visual summary compares the key particle terms used in early chemistry, including atoms, elements, molecules and compounds.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">4<\/div><h2>What Is a Compound?<\/h2><\/div>\n        <p>A <strong>compound<\/strong> is a substance containing atoms of two or more different elements chemically combined together.<\/p>\n        <p>Compounds have different properties from the elements that form them. For example, water, H<sub>2<\/sub>O, is a compound made from hydrogen and oxygen, but it does not behave like hydrogen gas or oxygen gas.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Exam focus:<\/strong> use the phrase <strong>chemically combined<\/strong> when defining a compound. A mixture is different because its substances are not chemically bonded together.<\/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: Which Particles Are Compounds<\/h2>\n<p>Click every formula that represents a molecule of a compound.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-846\" class=\"h5p-iframe\" data-content-id=\"846\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Molecules and Compounds Mark the Words: Which Particles Are Compounds\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft ols-zoom-parent\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">5<\/div><h2>What Is an Ion?<\/h2><\/div>\n        <p>An <strong>ion<\/strong> is a particle with an electric charge. It may be a single charged atom, such as Na<sup>+<\/sup>, or a group of atoms with an overall charge, such as SO<sub>4<\/sub><sup>2-<\/sup>.<\/p>\n        <p>A positively charged ion is called a <strong>cation<\/strong>. A negatively charged ion is called an <strong>anion<\/strong>.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Important:<\/strong> this page defines what an ion is and shows how ion charges are used in simple formulae. The formation of ions is covered in more detail when you study bonding and structure.<\/p><\/div>\n\n        <div class=\"ols-zoom-card\">\n          <div class=\"ols-figure-card\">\n<div class=\"ols-figure-image\"><img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/fix-63.jpg\" alt=\"Table linking a main-group element&#039;s group to the charge on its ion, with both the old and the modern group numbers\"><\/div>\n<div class=\"ols-figure-caption\"><p>Ion charges linked to periodic table groups for common metal and non-metal ions<\/p><\/div>\n<\/div>\n          <div class=\"ols-zoom-card-caption\">\n            <p>For many main-group ions, the group number helps predict the charge. Group 1 metals form +1 ions, Group 2 metals form +2 ions, Group 16 non-metals often form 2- ions and Group 17 non-metals often form 1- ions.<\/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: Reading an Ion<\/h2>\n<p>Count the atoms and work out the charge for each particle.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-847\" class=\"h5p-iframe\" data-content-id=\"847\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ions Quick Fire: Counting Atoms and Charges\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card purple ols-zoom-parent\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">6<\/div><h2>Working Out Formulae for Ionic Compounds<\/h2><\/div>\n        <p>You cannot write balanced equations accurately until you can write formulae. For ionic compounds, the key rule is that the total positive charge and total negative charge must be equal, because the compound is electrically neutral overall.<\/p>\n        <p>Start by writing the ions with their charges. Then choose the smallest whole-number ratio of ions that makes the charges cancel out.<\/p>\n\n        <div class=\"ols-rule-list\">\n          <div class=\"ols-rule-item\">\n            <h3>Sodium oxide<\/h3>\n            <p>Sodium forms Na<sup>+<\/sup> and oxygen forms O<sup>2-<\/sup>. Two sodium ions are needed for every oxide ion, so the formula is <strong>Na<sub>2<\/sub>O<\/strong>.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>Barium nitrate<\/h3>\n            <p>Barium forms Ba<sup>2+<\/sup> and nitrate is NO<sub>3<\/sub><sup>&#8211;<\/sup>. Two nitrate ions are needed for every barium ion, so the formula is <strong>Ba(NO<sub>3<\/sub>)<sub>2<\/sub><\/strong>.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>Iron(III) sulfate<\/h3>\n            <p>Iron(III) tells you the ion is Fe<sup>3+<\/sup>. Sulfate is SO<sub>4<\/sub><sup>2-<\/sup>. The lowest common charge total is 6, so two Fe<sup>3+<\/sup> ions and three SO<sub>4<\/sub><sup>2-<\/sup> ions are needed. The formula is <strong>Fe<sub>2<\/sub>(SO<sub>4<\/sub>)<sub>3<\/sub><\/strong>.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-zoom-card\">\n          <div class=\"ols-figure-card\">\n<div class=\"ols-figure-image\"><img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/fix-64.jpg\" alt=\"Balancing ion charges to write an ionic formula: sodium oxide, barium nitrate and iron(III) sulfate\"><\/div>\n<div class=\"ols-figure-caption\"><p>Step-by-step guide for determining ionic charges and writing ionic formulae<\/p><\/div>\n<\/div>\n          <div class=\"ols-zoom-card-caption\">\n            <p>This step-by-step visual shows how ion charges are balanced to produce the correct formula for an ionic compound.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-definition-box\"><p><strong>Bracket rule:<\/strong> use brackets when there is more than one polyatomic ion in the formula. For example, Ba(NO<sub>3<\/sub>)<sub>2<\/sub> needs brackets because there are two nitrate ions.<\/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: Build Seven Formulae<\/h2>\n<p>Write each formula on paper first, then flip the card to check the ratio and the brackets.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-284\" class=\"h5p-iframe\" data-content-id=\"284\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Writing Formulae Flip Cards: Balancing the Charges with Brackets\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card orange ols-zoom-parent\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">7<\/div><h2>Why Ion Charges Are Not Usually Written in Formulae<\/h2><\/div>\n        <p>Ion charges can be shown when you are explaining how a formula is constructed, such as Na<sup>+<\/sup> and Cl<sup>&#8211;<\/sup>. However, in the final formula of an ionic compound, the charges are normally omitted.<\/p>\n        <p>For example, sodium chloride is written as <strong>NaCl<\/strong>, not Na<sup>+<\/sup>Cl<sup>&#8211;<\/sup>, because the formula represents the electrically neutral compound. The charges are still there in the ions, but they are not shown in the final formula.<\/p>\n\n        <div class=\"ols-zoom-card\">\n          <div class=\"ols-zoom-card-image\">\n            <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/wave-31.jpg\" alt=\"Essential ions cheat sheet with sulfate spelled the way the boards spell it.\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n          <\/div>\n          <div class=\"ols-zoom-card-caption\">\n            <p>Use common ion charges carefully when constructing formulae. The final formula must be neutral overall, even though the individual ions are charged.<\/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: Spot the Accurate Statement<\/h2>\n<p>In each round, choose the one statement that is accurate.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-848\" class=\"h5p-iframe\" data-content-id=\"848\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Atoms and Formulae Summary: Spot the Accurate Statement\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">8<\/div><h2>Monatomic, Diatomic and Polyatomic Terms<\/h2><\/div>\n        <p>These words describe how many atoms are present in a particle. They can be used for elements, molecules and ions.<\/p>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Term<\/th>\n                <th>Meaning<\/th>\n                <th>Examples<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Term\">Monatomic<\/td>\n                <td data-label=\"Meaning\">Made from single atoms or single-atom ions.<\/td>\n                <td data-label=\"Examples\">He, Ne, Cl<sup>&#8211;<\/sup><\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Term\">Diatomic<\/td>\n                <td data-label=\"Meaning\">Made from two atoms joined together, or an ion containing two atoms.<\/td>\n                <td data-label=\"Examples\">H<sub>2<\/sub>, N<sub>2<\/sub>, O<sub>2<\/sub>, OH<sup>&#8211;<\/sup><\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Term\">Polyatomic<\/td>\n                <td data-label=\"Meaning\">Made from several atoms joined together.<\/td>\n                <td data-label=\"Examples\">P<sub>4<\/sub>, CH<sub>4<\/sub>, SO<sub>4<\/sub><sup>2-<\/sup><\/td>\n              <\/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: The Words for Particles<\/h2>\n<p>Fit the right term into the grid for each description.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-849\" class=\"h5p-iframe\" data-content-id=\"849\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Particle Words Crossword: Element, Ion or Mixture\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">9<\/div><h2>Common Exam Points<\/h2><\/div>\n        <ul>\n          <li>An element contains <strong>only one type of atom<\/strong>.<\/li>\n          <li>A compound contains atoms of <strong>two or more different elements chemically combined<\/strong>.<\/li>\n          <li>A molecule contains <strong>two or more atoms bonded together<\/strong>.<\/li>\n          <li>H<sub>2<\/sub> is a molecule of an element because it contains only hydrogen atoms.<\/li>\n          <li>H<sub>2<\/sub>O is a molecule of a compound because it contains hydrogen and oxygen atoms.<\/li>\n          <li>An ion has an <strong>overall positive or negative charge<\/strong>.<\/li>\n          <li>A cation is positive. An anion is negative.<\/li>\n          <li>Ionic compounds are neutral overall, so the total positive charge must equal the total negative charge.<\/li>\n          <li>Use brackets when there is more than one polyatomic ion, such as Ba(NO<sub>3<\/sub>)<sub>2<\/sub> or Fe<sub>2<\/sub>(SO<sub>4<\/sub>)<sub>3<\/sub>.<\/li>\n          <li>Do not confuse chemical symbols with formulae. <strong>O<\/strong> is oxygen as an element symbol, while <strong>O<sub>2<\/sub><\/strong> is an oxygen molecule.<\/li>\n        <\/ul>\n      <\/article>\n\n      \n\n      <section class=\"ols-quicksnap-card\">\n        <h2>QuickSnap<\/h2>\n        <p>This text summary condenses the page into the essential exam ideas.<\/p>\n        <ul class=\"ols-quicksnap-list\">\n          <li><strong>Element:<\/strong> a substance made from only one type of atom.<\/li>\n          <li><strong>Atom:<\/strong> the smallest particle of an element that retains that element&#8217;s chemical identity.<\/li>\n          <li><strong>Molecule:<\/strong> two or more atoms chemically bonded together.<\/li>\n          <li><strong>Compound:<\/strong> a substance containing atoms of different elements chemically combined.<\/li>\n          <li><strong>Ion:<\/strong> a charged particle. Positive ions are cations and negative ions are anions.<\/li>\n          <li><strong>Ionic formulae:<\/strong> total positive charge must equal total negative charge because ionic compounds are neutral overall.<\/li>\n          <li><strong>Brackets:<\/strong> use brackets when a formula contains more than one polyatomic ion, such as Ba(NO<sub>3<\/sub>)<sub>2<\/sub>.<\/li>\n          <li><strong>Formulae:<\/strong> H<sub>2<\/sub> shows two hydrogen atoms in one hydrogen molecule, while H<sub>2<\/sub>O shows two hydrogen atoms and one oxygen atom in water.<\/li>\n        <\/ul>\n      <\/section>\n\n      <div class=\"ols-image-lightbox\" id=\"olsImageLightboxAtomsElementsMolecules\" aria-hidden=\"true\" role=\"dialog\" aria-label=\"Expanded image viewer\">\n        <button class=\"ols-image-lightbox-close\" type=\"button\" aria-label=\"Close expanded image\">\u00d7<\/button>\n        <div class=\"ols-image-lightbox-inner\">\n          <img decoding=\"async\" class=\"ols-image-lightbox-img\" src=\"\" alt=\"\">\n        <\/div>\n      <\/div>\n\n      <script>\n        (function(){\n          var page = document.querySelector('.ols-atoms-elements-molecules-page');\n          if (!page) {\n            return;\n          }\n\n          var lightbox = page.querySelector('#olsImageLightboxAtomsElementsMolecules');\n          if (!lightbox) {\n            return;\n          }\n\n          var lightboxImage = lightbox.querySelector('.ols-image-lightbox-img');\n          var closeButton = lightbox.querySelector('.ols-image-lightbox-close');\n          var targets = page.querySelectorAll('img.ols-lightbox-target');\n\n          function openLightbox(image) {\n            if (!image || !lightboxImage) {\n              return;\n            }\n\n            lightboxImage.src = image.currentSrc || image.src;\n            lightboxImage.alt = image.alt || '';\n            lightbox.classList.add('is-open');\n            lightbox.setAttribute('aria-hidden', 'false');\n            document.body.classList.add('ols-lightbox-lock');\n          }\n\n          function closeLightbox() {\n            lightbox.classList.remove('is-open');\n            lightbox.setAttribute('aria-hidden', 'true');\n            document.body.classList.remove('ols-lightbox-lock');\n            if (lightboxImage) {\n              lightboxImage.src = '';\n              lightboxImage.alt = '';\n            }\n          }\n\n          targets.forEach(function(image){\n            image.addEventListener('click', function(event){\n              event.preventDefault();\n              event.stopPropagation();\n              openLightbox(image);\n            });\n          });\n\n          if (closeButton) {\n            closeButton.addEventListener('click', closeLightbox);\n          }\n\n          lightbox.addEventListener('click', function(event){\n            if (event.target === lightbox) {\n              closeLightbox();\n            }\n          });\n\n          document.addEventListener('keydown', function(event){\n            if (event.key === 'Escape' && lightbox.classList.contains('is-open')) {\n              closeLightbox();\n            }\n          });\n        })();\n      <\/script>\n\n      <section class=\"ols-faq-card\">\n        <h2>FAQs<\/h2>\n        <p>Use these questions to secure the wording needed for written definitions and short exam responses.<\/p>\n        <div class=\"ols-faq-list\">\n          <div class=\"ols-faq-item\">\n            <h3>What is the difference between an atom and an element?<\/h3>\n            <p>An atom is a single particle. An element is a substance made from only one type of atom. For example, oxygen is an element, and one oxygen atom is one particle of that element.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>What is the difference between a molecule and a compound?<\/h3>\n            <p>A molecule is two or more atoms bonded together. A compound contains atoms of two or more different elements chemically combined. Some molecules are compounds, such as H<sub>2<\/sub>O, but some molecules are elements, such as H<sub>2<\/sub>.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>Is H<sub>2<\/sub> an element or a compound?<\/h3>\n            <p>H<sub>2<\/sub> is a molecule of an element. It contains two hydrogen atoms bonded together, but both atoms are the same element.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>Why is H<sub>2<\/sub>O a compound?<\/h3>\n            <p>H<sub>2<\/sub>O is a compound because it contains atoms of two different elements, hydrogen and oxygen, chemically bonded together.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>What is the difference between a cation and an anion?<\/h3>\n            <p>A cation is a positively charged ion. An anion is a negatively charged ion. For example, Na<sup>+<\/sup> is a cation and Cl<sup>&#8211;<\/sup> is an anion.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>How do you work out the formula of an ionic compound?<\/h3>\n            <p>Write the ions with their charges, then choose the smallest whole-number ratio that makes the total positive charge equal the total negative charge.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>When do you use brackets in an ionic formula?<\/h3>\n            <p>Use brackets when there is more than one polyatomic ion. For example, barium nitrate is Ba(NO<sub>3<\/sub>)<sub>2<\/sub> because there are two nitrate ions.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>Why are ion charges not usually shown in final formulae?<\/h3>\n            <p>The final formula represents an electrically neutral compound, so the charges are not normally written even though the particles present are ions.<\/p>\n          <\/div>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-related-card\">\n        <h2>Related Topics<\/h2>\n        <p>Continue through this section 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\/ocr-a\/2-1-2-compounds-formulae-and-equations\/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\/ocr-a\/2-1-2-compounds-formulae-and-equations\/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\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/\">Mole Calculations<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/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\/ocr-a\/2-1-3-amount-of-substance\/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\/ocr-a\/2-1-1-atomic-structure-and-isotopes\/\">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. 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