Atomic number | Definition & Facts

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They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> The Editors of Encyclopaedia Britannica</span></div> <div class="last-updated font-12 font-serif"> <span class="text-gray-700"> Last Updated: <time datetime="2024-11-22T00:00:00CST" >Nov 22, 2024</time> •</span> <a class="byline-edit-history" href="https://www.britannica.com/science/atomic-number/additional-info#history" rel="nofollow">Article History</a> </div></div> </div> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button d-none d-sm-block js-sections-inline-button module-spacing btn d-lg-none"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <div class="d-flex d-sm-none flex-row"> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button js-sections-inline-button module-spacing"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <button class="ai-ask-button btn border-2 ai-ask-button btn border-2 module-spacing btn-sm js-inline-ai-ask-button btn-outline-red-400 border-red-400 p-10 ml-5"> Ask the Chatbot a Question </button> </div> <div class="js-qf-module qf-module px-40 px-sm-20 py-15 mx-auto module-spacing font-14 bg-gray-50 rounded"> <div class="facts-list mt-10"> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Key People: </dt> <dd><a href="/biography/Henry-Moseley" topicid="393528">Henry Moseley</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> </div> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Related Topics: </dt> <dd><a href="/science/atom" topicid="41549">atom</a></dd> <dd><a href="/science/Moseleys-law" topicid="393536">Moseley’s law</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> </div> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>On the Web: </dt> <dd><a href="https://www.nature.com/articles/s41598-023-30491-5" target="_blank">Nature - Investigation of the effective atomic number dependency on kinetic energy using collision stopping powers for electrons, protons, alpha, and carbon particles</a> (Nov. 22, 2024)</dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> <div class="text-center"> <a class="btn btn-sm btn-link p-0" href="/facts/atomic-number"> See all related content </a> </div> </div> </div> </div><span class="marker before-article"></span><section data-level="1" id="ref1"><span class="marker PREMOD1 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="280881" data-asm-type="video"><div class="md-assembly-wrapper card-media " data-type="video" video-id="240879"><a data-id="240879" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/How-Can-You-See-an-Atom/-280881"><img src="https://cdn.britannica.com/79/240879-138-C83919AE/How-Can-You-See-an-Atom.jpg?w=800&h=450&c=crop" alt="atoms" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/6whHYWxD.m3u8" } ], "image": "https://cdn.britannica.com/79/240879-138-C83919AE/How-Can-You-See-an-Atom.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/YtN2sgKk", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/PgXWZv1s.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="240879">atoms</span><span>How atoms can be seen.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/atomic-number/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph"><strong><span id="ref22135"></span>atomic number</strong>, the number of a <a href="https://www.britannica.com/science/chemical-element" class="md-crosslink " data-show-preview="true">chemical element</a> in the periodic system and on the <a href="https://www.britannica.com/science/periodic-table" class="md-crosslink " data-show-preview="true">periodic table</a> that equals the number of protons in the nucleus of the atom. The elements are arranged on the table in order of increasing number of protons in the nucleus. Accordingly, the number of <a href="https://www.britannica.com/science/proton-subatomic-particle" class="md-crosslink " data-show-preview="true">protons</a>, which is always equal to the number of <a href="https://www.britannica.com/science/electron" class="md-crosslink " data-show-preview="true">electrons</a> in the neutral atom, is also the atomic number. An <a href="https://www.britannica.com/science/atom" class="md-crosslink " data-show-preview="true">atom</a> of <span id="ref1275083"></span><a href="https://www.britannica.com/science/iron-chemical-element" class="md-crosslink " data-show-preview="true">iron</a> has 26 protons in its nucleus; therefore, the atomic number of iron is 26 and its ranking on the periodic table of chemical elements is 26.</p><div class="one-good-fact-module"> </div><span class="marker MOD1 mod-inline"></span><span class="marker PREMOD2 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="162921" data-asm-type="video"><div class="md-assembly-wrapper card-media " data-type="video" video-id="152190"><a data-id="152190" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/Description-arrangement-electrons-atoms-elements/-162921"><img src="https://cdn.britannica.com/90/152190-138-47A9DC74/Description-arrangement-electrons-atoms-elements.jpg?w=800&h=450&c=crop" alt="A simplified look into the structure of an atom" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/gKqhQCSr.m3u8" } ], "image": "https://cdn.britannica.com/90/152190-138-47A9DC74/Description-arrangement-electrons-atoms-elements.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/5qcqd4u3", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/EncAYN7P.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="152190">A simplified look into the structure of an atom</span><span>Explore an atom's interior to discover the layout of its nucleus, protons, and electrons.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/atomic-number/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">In the symbol representing a particular nuclear or atomic species, the atomic number may be indicated as a left subscript. An atom or a nucleus of iron (chemical symbol Fe), for example, may be written <sub>2</sub><sub>6</sub>Fe.</p><span class="marker MOD2 mod-inline"></span></section> <span class="md-signature"><a href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419">The Editors of Encyclopaedia Britannica</a></span><span class="md-signature">This article was most recently revised and updated by <a href="/editor/Adam-Augustyn/6394">Adam Augustyn</a>.</span><span class="marker end-of-content"></span><span class="marker after-article"></span></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article></div> </div></div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript> <script type="text/javascript" id="_informizely_script_tag"> var IzWidget = IzWidget || {}; (function (d) { var scriptElement = d.createElement('script'); scriptElement.type = 'text/javascript'; scriptElement.async = true; scriptElement.src = "https://insitez.blob.core.windows.net/site/f780f33e-a610-4ac2-af81-3eb184037547.js"; var node = d.getElementById('_informizely_script_tag'); node.parentNode.insertBefore(scriptElement, node); } )(document); </script>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":1,"pagesTotal":1,"pageId":41745,"pageLength":135,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-130/js/libs/jquery-3.5.0.min.js?v=3.130.14"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-130/dist/topic-page.js?v=3.130.14' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"A","adLeg":"A","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"SMALL","gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":false} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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