What is a radioactive isotope?

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Let us know if you have suggestions to improve this article (requires login). </div> <div class="type-menu"> <label for="feedback-type" class="label mb-10">Feedback Type</label> <select id="feedback-type" class="form-select mb-30" name="feedbackTypeId" required> <option value="" selected="selected">Select a type (Required)</option> <option value="1">Factual Correction</option> <option value="2">Spelling/Grammar Correction</option> <option value="3">Link Correction</option> <option value="4">Additional Information</option> <option value="5">Other</option> </select> </div> <label for="feedback" class="label mb-10">Your Feedback</label> <textarea id="feedback" class="form-control mb-30" name="feedback" maxlength="3000" rows="7" required></textarea> <button class="btn btn-blue" type="submit">Submit Feedback</button> </form> <div class="success-messaging d-none mt-30"> <div class="title">Thank you for your feedback</div> <p>Our editors will review what you’ve submitted and determine whether to revise the article.</p> </div> </div> </div> <div class="md-websites-modal size-lg d-none"> <div class="md-modal-body"> <div class="h2 font-serif pb-15 border-bottom font-weight-bold"> External Websites </div> </div> </div> </div> </div> <div class="md-byline module-spacing "> <div class="font-serif font-12"> <span class="written-by text-gray-700"> Written and fact-checked by </span> <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup gtm-byline font-12 byline-contributor" href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419" > <div class="editor-title font-16 font-weight-bold">The Editors of Encyclopaedia Britannica</div> <div class="editor-description font-12 font-serif mt-5 text-black">Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. 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="2025-02-13T00:00:00CST" >Feb 13, 2025</time> </span> </div></div> <div class="mt-20"> <div class="collection-content topic-paragraph"> <article class="content"><div class="font-serif"><p>A radioactive isotope, also known as a radioisotope, radionuclide, or radioactive nuclide, is any of several species of the same <a href="https://www.britannica.com/science/chemical-element" class="md-crosslink " data-show-preview="true">chemical element</a> with different masses whose nuclei are unstable and dissipate excess energy by spontaneously emitting <a href="https://www.britannica.com/science/radiation" class="md-crosslink " data-show-preview="true">radiation</a> in the form of <a href="https://www.britannica.com/science/alpha-particle" class="md-crosslink " data-show-preview="true">alpha</a>, <a href="https://www.britannica.com/science/beta-particle" class="md-crosslink " data-show-preview="true">beta</a>, and <a href="https://www.britannica.com/science/gamma-ray" class="md-crosslink " data-show-preview="true">gamma</a> rays. Every chemical element has one or more radioactive isotopes. For example, <a href="https://www.britannica.com/science/hydrogen" class="md-crosslink " data-show-preview="true">hydrogen</a>, the lightest element, has three isotopes, which have mass numbers 1, 2, and 3. Only hydrogen-3 (<a href="https://www.britannica.com/science/tritium" class="md-crosslink " data-show-preview="true">tritium</a>), however, is a radioactive isotope; the other two are stable. More than 1,800 radioactive isotopes of the various elements are known. Some of these are found in nature; the rest are produced artificially as the direct products of nuclear reactions or indirectly as the radioactive descendants of these products. Each “parent” radioactive isotope eventually decays into one or at most a few stable isotope “daughters” specific to that parent.</p></div></article></div> </div> <div class="related-questions mb-30"> <h4 class="mb-15">Related Questions</h4> <ul class="list-unstyled"> <li class=""> <a class="p-15 mb-10 rounded text-black font-weight-semi-bold d-block" href="/question/How-are-radioactive-isotopes-produced">How are radioactive isotopes produced?</a> </li> <li class=""> <a class="p-15 mb-10 rounded text-black font-weight-semi-bold d-block" href="/question/How-are-radioactive-isotopes-used-in-medicine">How are radioactive isotopes used in medicine?</a> </li> </ul> </div> </div> </div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </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":"TopicTopQuestionsPage","templateName":"TOP_QUESTION","pageNumber":1,"pagesTotal":2,"pageId":null,"pageLength":null,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-133/js/libs/jquery-3.5.0.min.js?v=3.133.9"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-133/dist/topic-top-questions-page.js?v=3.133.9' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"C","adLeg":"C","userType":"ANONYMOUS","pageType":"TopicTopQuestionsPage","pageSubtype":"TOP_QUESTION","articleTemplateType":null,"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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What is a radioactive isotope? | Britannica