Oxygen | Discovery, Symbol, Properties, Uses, & Facts

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position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/science/oxygen">oxygen</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/science/oxygen">Introduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref279400"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/oxygen#ref279400">History</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref279401"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/oxygen#ref279401">Occurrence and properties</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref279402"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/oxygen#ref279402">Allotropy</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref279403"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/oxygen#ref279403">Preparative methods</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref279404"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/oxygen#ref279404">Commercial production and use</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref279405"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/oxygen#ref279405">Chemical properties and reactions</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/science/oxygen/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/oxygen">Quick Facts & Related Topics</a> </div> <div class="tlr-media-slider pb-10 mb-30"> <a class="section-header link-gray-900 font-serif font-14 font-weight-bold mb-10 mx-10" href="https://www.britannica.com/science/oxygen/images-videos">Images, Videos & Interactives</a> <div class="slider js-slider position-relative d-inline-flex align-items-center mw-100 "> <div class="slider-container js-slider-container overflow-hidden d-flex overflow-hidden text-nowrap ml-15"> <a href="https://cdn.britannica.com/92/22392-050-148E8CC1/properties-weight-chemical-element-crystal-structure-atomic.jpg" data-href="/media/1/436806/64459" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/92/22392-004-D10DB3F1/properties-weight-chemical-element-crystal-structure-atomic.jpg" alt="oxygen" height="50" /> </a> <a href="/video/How-much-air-do-people-breathe-in-a-lifetime/301494" data-href="/media/1/436806/301494" class="media-overlay-link d-inline-block mr-5"> <div class="position-relative --aspect-ratio: 16/9"> <img loading="lazy" src="https://cdn.britannica.com/96/253596-138-3435681B/How-much-air-do-people-breathe-in-a-lifetime.jpg?w=400&h=225&c=crop" alt="How much air do you breathe in a lifetime?" class="col-100" /> <div class="btn btn-sm btn-white btn-circle position-absolute shadow" style="top: 50%; left:50%; transform: translate(-50%, -50%)"> <em class="material-icons font-14" data-icon="play_arrow" > </em> </div> </div> </a> <a href="" data-href="/media/1/436806/299687" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/13/255013-049-6477CC72/periodic-table-banner.jpg" alt="Explore an interactive periodic table of the elements" height="50" /> </a> <a href="https://cdn.britannica.com/65/7465-004-4ABF79AE/Explosion-limits-hydrogen-oxygen-mixture.jpg" data-href="/media/1/436806/925" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/65/7465-004-4ABF79AE/Explosion-limits-hydrogen-oxygen-mixture.jpg" alt="explosion limits" height="50" /> </a> <a href="https://cdn.britannica.com/13/6013-050-345F408D/guess-abundance-reconstruction-O-atmosphere-function-Earth.jpg" data-href="/media/1/436806/619" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/13/6013-004-B9387004/guess-abundance-reconstruction-O-atmosphere-function-Earth.jpg" alt="abundance of oxygen" height="50" /> </a> <a href="https://cdn.britannica.com/78/126078-050-6EA71704/oxygen-cycle.jpg" data-href="/media/1/436806/126790" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/78/126078-004-0AEECCB0/oxygen-cycle.jpg" alt="oxygen cycle" height="50" /> </a> <a href="https://cdn.britannica.com/23/6023-050-C4B4B3BC/view-ozone-chemistry-oxygen-environment-Ultraviolet-light.jpg" data-href="/media/1/436806/626" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/23/6023-004-5914F2B7/view-ozone-chemistry-oxygen-environment-Ultraviolet-light.jpg" alt="Figure 10: Schematic view of ozone chemistry in a pure oxygen environment." height="50" /> </a> <a href="https://cdn.britannica.com/04/96904-050-140D0BAE/atoms-bond-bonds-electrons-hydrogen-oxygen-atom.jpg" data-href="/media/1/436806/92140" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/04/96904-004-98113EF2/atoms-bond-bonds-electrons-hydrogen-oxygen-atom.jpg" alt="polar covalent bond" height="50" /> </a> <a href="https://cdn.britannica.com/68/105668-050-6FB3401B/temperature-range.jpg" data-href="/media/1/436806/147313" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/68/105668-004-D2B8F7DA/temperature-range.jpg" alt="cryogenic region" height="50" /> </a> <a href="https://cdn.britannica.com/92/1592-004-D96A672B/oxygen-sensor-diagram-automobile-exhaust-gases-air.jpg" data-href="/media/1/436806/265" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/92/1592-004-D96A672B/oxygen-sensor-diagram-automobile-exhaust-gases-air.jpg" alt="Figure 1: Schematic diagram of a zirconia oxygen sensor used to monitor automobile exhaust gases. The sensor, approximately the size of a spark plug, is fitted into the exhaust manifold of an automobile engine. The thimble-shaped zirconia sensor, sandwiched between thin layers of porous platinum, is exposed on its interior to outside air and on its exterior to exhaust gas passing through slits in the sensor shield. The two platinum surfaces serve as electrodes, conducting a voltage across the zirconia that varies according to the difference in oxygen content between the exhaust gas and the outside air." height="50" /> </a> </div> <button disabled class="prev-button js-prev-button position-absolute btn btn-circle shadow btn-blue " aria-label="Previous"> <span class="material-icons md-24" data-icon="keyboard_arrow_left"></span> </button> <button disabled class="next-button js-next-button position-absolute btn btn-circle shadow btn-blue " aria-label="Next"> <span class="material-icons md-24" data-icon="keyboard_arrow_right"></span> </button> </div> </div> <div class="mb-30 tlr-student-links"> <div class="text-gray-900 p-5 font-serif font-14 font-weight-bold mx-10 mb-10"> For Students </div> <div class="imagelink-with-image-on-the-side card card-horizontal tlr-img-with-side-link ml-15 link-gray-900 mb-10" > <div class="position-relative card-media" style="flex: 0;"> <a class="ilf-image position-relative" href="/summary/oxygen"> <img loading="lazy" src="https://cdn.britannica.com/mendel-resources/3-134/images/shared/default3.png?v=3.134.9" class="default " height="200" width="200"/> </a> </div> <div class="card-body ilf-content"> <a class="font-weight-semi-bold d-block mb-5 font-16 ilf-title" href="/summary/oxygen" >oxygen summary</a> </div> </div> </div> <div class="mb-30 tlr-related-quizzes"> <div class="text-gray-900 p-5 font-serif font-14 font-weight-bold mx-10 mb-10"> Quizzes </div> <div class="imagelink-with-image-on-the-side card card-horizontal tlr-img-with-side-link ml-15 link-gray-900 mb-10" > <div class="position-relative card-media" style="flex: 0;"> <a class="ilf-image position-relative" href="/quiz/periodic-table-names-symbols-quiz"> <img loading="lazy" src="https://cdn.britannica.com/66/215466-131-16036BD8/Concept-artwork-periodic-table-elements.jpg?w=200&h=200&c=crop" alt="Concept artwork on the periodic table of elements." width="200" height="200" /> </a> </div> <div class="card-body ilf-content"> <a class="font-weight-semi-bold d-block mb-5 font-16 ilf-title" href="/quiz/periodic-table-names-symbols-quiz" >118 Names and Symbols of the Periodic Table Quiz</a> </div> </div> <div class="imagelink-with-image-on-the-side card card-horizontal tlr-img-with-side-link ml-15 link-gray-900 mb-10" > <div class="position-relative card-media" style="flex: 0;"> <a class="ilf-image position-relative" href="/quiz/facts-you-should-know-the-periodic-table-quiz"> <img loading="lazy" src="https://cdn.britannica.com/58/203458-131-D94E9327/periodic-table-concept.jpg?w=200&h=200&c=crop" alt="Periodic Table of the elements concept image (chemistry)" width="200" height="200" /> </a> </div> <div class="card-body ilf-content"> <a class="font-weight-semi-bold d-block mb-5 font-16 ilf-title" href="/quiz/facts-you-should-know-the-periodic-table-quiz" >Facts You Should Know: The Periodic Table Quiz</a> </div> </div> <div class="imagelink-with-image-on-the-side card card-horizontal tlr-img-with-side-link ml-15 link-gray-900 mb-10" > <div class="position-relative card-media" style="flex: 0;"> <a class="ilf-image position-relative" href="/quiz/27-true-or-false-questions-from-britannicas-most-difficult-science-quizzes"> <img loading="lazy" src="https://cdn.britannica.com/05/215905-131-626D860A/Encyclopaedia-Britannica-thistle-graphic-logo.jpg?w=200&h=200&c=crop" alt="Encyclopaedia Britannica thistle graphic to be used with a Mendel/Consumer quiz in place of a photograph." width="200" height="200" /> </a> </div> <div class="card-body ilf-content"> <a class="font-weight-semi-bold d-block mb-5 font-16 ilf-title" href="/quiz/27-true-or-false-questions-from-britannicas-most-difficult-science-quizzes" >27 True-or-False Questions from Britannica’s Most Difficult Science Quizzes</a> </div> </div> <div class="imagelink-with-image-on-the-side card card-horizontal tlr-img-with-side-link ml-15 link-gray-900 mb-10" > <div class="position-relative card-media" style="flex: 0;"> <a class="ilf-image position-relative" href="/quiz/so-much-chemistry-so-little-time-quiz"> <img loading="lazy" src="https://cdn.britannica.com/16/150616-131-320CDF08/Illustration-molecules.jpg?w=200&h=200&c=crop" alt="Illustration of molecules. 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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 font-12 "> The Editors of Encyclopaedia Britannica</span></div> <div class="last-updated font-12 font-serif"> <span class="text-gray-700"> Last Updated: <time datetime="2025-02-14T00:00:00CST" >Feb 14, 2025</time> •</span> <a class="byline-edit-history" href="https://www.britannica.com/science/oxygen/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 btn-sm border-2 btn-outline-red-400 border-red-400 module-spacing js-inline-ai-ask-button p-10 ml-5"> Ask the Chatbot </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/Carl-Paul-Gottfried-von-Linde" topicid="341835">Carl von Linde</a></dd> <dd><a href="/biography/Antoine-Lavoisier" topicid="332700">Antoine Lavoisier</a></dd> <dd><a href="/biography/Joseph-Priestley" topicid="475975">Joseph Priestley</a></dd> <dd><a href="/biography/Carl-Wilhelm-Scheele" topicid="527125">Carl Wilhelm Scheele</a></dd> <dd><a href="/biography/John-Mayow" topicid="371088">John Mayow</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/ozone" topicid="437194">ozone</a></dd> <dd><a href="/science/oxygen-17" topicid="436942">oxygen-17</a></dd> <dd><a href="/science/oxygen-18" topicid="436944">oxygen-18</a></dd> <dd><a href="/science/liquid-oxygen" topicid="343157">liquid oxygen</a></dd> <dd><a href="/science/oxygen-16" topicid="436939">oxygen-16</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 class="text-center"> <a class="btn btn-sm btn-link p-0" href="/facts/oxygen"> 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><p class="topic-paragraph"><strong><span id="ref252238"></span>oxygen (O)</strong>, nonmetallic <a href="https://www.britannica.com/science/chemical-element" class="md-crosslink autoxref " data-show-preview="true">chemical element</a> of Group 16 (VIa, or the <a href="https://www.britannica.com/science/oxygen-group-element" class="md-crosslink " data-show-preview="true">oxygen group</a>) of the <a href="https://www.britannica.com/science/periodic-table" class="md-crosslink autoxref " data-show-preview="true">periodic table</a>. Oxygen is a colourless, odourless, tasteless <a href="https://www.britannica.com/science/gas-state-of-matter" class="md-crosslink autoxref " data-show-preview="true">gas</a> essential to living organisms, being taken up by animals, which convert it to <a href="https://www.britannica.com/science/carbon-chemical-element" class="md-crosslink autoxref " data-show-preview="true">carbon</a> dioxide; plants, in turn, utilize <a href="https://www.britannica.com/science/carbon-dioxide" class="md-crosslink autoxref " data-show-preview="true">carbon dioxide</a> as a source of carbon and return the oxygen to the atmosphere. Oxygen forms <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="compounds" href="https://www.merriam-webster.com/dictionary/compounds" data-type="MW">compounds</a> by reaction with practically any other element, as well as by reactions that displace elements from their combinations with each other; in many cases, these processes are accompanied by the evolution of heat and light and in such cases are called combustions. Its most important <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="compound" href="https://www.merriam-webster.com/dictionary/compound" data-type="MW">compound</a> is <a href="https://www.britannica.com/science/water" class="md-crosslink autoxref " data-show-preview="true">water</a>.</p><span class="marker MOD1 mod-inline"></span><div class="text-center pb-20"><div class="md-table-wrapper"><table class="md-element-table"><caption>Element Properties</caption><tbody><tr><th scope="row">atomic number</th><td>8</td></tr><tr><th scope="row">atomic weight</th><td>15.9994</td></tr><tr><th scope="row">melting point</th><td>−218.4 °C (−361.1 °F)</td></tr><tr><th scope="row">boiling point</th><td>−183.0 °C (−297.4 °F)</td></tr><tr><th scope="row">density (1 atm, 0 °C)</th><td>1.429 g/litre</td></tr><tr><th scope="row">oxidation states</th><td>−1, −2, +2 (in compounds with fluorine)</td></tr><tr><th scope="row">electron config.</th><td>1<em>s</em><sup>2</sup>2<em>s</em><sup>2</sup>2<em>p</em><sup>4</sup></td></tr></tbody></table></div></div></section> <span class="marker h2"></span><section data-level="1" id="ref279400"> <h2 class="h1">History</h2> <span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">Oxygen was discovered about 1772 by a Swedish chemist, <a href="https://www.britannica.com/biography/Carl-Wilhelm-Scheele" class="md-crosslink " data-show-preview="true">Carl Wilhelm Scheele</a>, who obtained it by heating <a href="https://www.britannica.com/science/potassium-nitrate" class="md-crosslink autoxref " data-show-preview="true">potassium nitrate</a>, mercuric <a href="https://www.britannica.com/science/oxide" class="md-crosslink autoxref " data-show-preview="true">oxide</a>, and many other substances. An English chemist, <span id="ref252239"></span><a href="https://www.britannica.com/biography/Joseph-Priestley" class="md-crosslink " data-show-preview="true">Joseph Priestley</a>, independently discovered oxygen in 1774 by the thermal decomposition of mercuric oxide and published his findings the same year, three years before Scheele published. In 1775–80, French chemist <span id="ref252240"></span><a href="https://www.britannica.com/biography/Antoine-Lavoisier" class="md-crosslink " data-show-preview="true">Antoine-Laurent Lavoisie</a>r, with remarkable insight, interpreted the role of oxygen in respiration as well as <a href="https://www.britannica.com/science/combustion" class="md-crosslink autoxref " data-show-preview="true">combustion</a>, discarding the <a href="https://www.britannica.com/science/phlogiston" class="md-crosslink autoxref " data-show-preview="true">phlogiston</a> theory, which had been accepted up to that time; he noted its tendency to form acids by <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="combining" href="https://www.britannica.com/dictionary/combining" data-type="EB">combining</a> with many different substances and accordingly named the element <em>oxygen</em> (<em>oxygène</em>) from the Greek words for “acid former.”</p><span class="marker MOD2 mod-inline"></span> </section> <span class="marker h3"></span><section data-level="1" id="ref279401"> <h2 class="h1">Occurrence and properties</h2> <span class="marker PREMOD3 mod-inline"></span><p class="topic-paragraph">At 46 percent of the mass, oxygen is the most plentiful element in <a href="https://www.britannica.com/place/Earth" class="md-crosslink autoxref " data-show-preview="true">Earth’s</a> crust. The proportion of oxygen by volume in the atmosphere is 21 percent and by weight in <a href="https://www.britannica.com/science/seawater" class="md-crosslink autoxref " data-show-preview="true">seawater</a> is 89 percent. In rocks, it is combined with metals and nonmetals in the form of oxides that are acidic (such as those of <a href="https://www.britannica.com/science/sulfur" class="md-crosslink autoxref " data-show-preview="true">sulfur</a>, carbon, <a href="https://www.britannica.com/science/aluminum" class="md-crosslink autoxref " data-show-preview="true">aluminum</a>, and phosphorus) or basic (such as those of <a href="https://www.britannica.com/science/calcium" class="md-crosslink autoxref " data-show-preview="true">calcium</a>, <a href="https://www.britannica.com/science/magnesium" class="md-crosslink autoxref " data-show-preview="true">magnesium</a>, and iron) and as saltlike compounds that may be regarded as formed from the acidic and basic oxides, as sulfates, carbonates, silicates, aluminates, and phosphates. Plentiful as they are, these solid compounds are not useful as sources of oxygen, because separation of the element from its tight combinations with the <a href="https://www.britannica.com/science/metal-chemistry" class="md-crosslink autoxref " data-show-preview="true">metal</a> atoms is too expensive.</p><a class="link-module shadow-sm d-block qa-quiz-module" href="/quiz/periodic-table-names-symbols-quiz" data-link-module-iframe-link=""> <img loading="lazy" src="https://cdn.britannica.com/66/215466-131-16036BD8/Concept-artwork-periodic-table-elements.jpg" alt="Concept artwork on the periodic table of elements." class="rounded-sm mr-15" width="70" /> <div class="line-clamp clamp-5"> <div class="module-title bg-green">Britannica Quiz</div> <div class="font-weight-semi-bold mt-5">118 Names and Symbols of the Periodic Table Quiz</div> </div> </a><span class="marker MOD3 mod-inline"></span> <span class="marker PREMOD4 mod-inline"></span><p class="topic-paragraph">Below −183 <a href="https://www.britannica.com/technology/Celsius-temperature-scale" class="md-crosslink autoxref " data-show-preview="true">°C</a> (−297 °F), oxygen is a pale blue liquid; it becomes solid at about −218 °C (−361 °F). Pure oxygen is 1.1 times heavier than <a href="https://www.britannica.com/science/air" class="md-crosslink autoxref " data-show-preview="true">air</a>.</p><span class="marker MOD4 mod-inline"></span> <span class="marker PREMOD5 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="301494" data-asm-type="video"><div class="md-assembly-wrapper card-media" data-type="video" video-id="253596"><a data-id="253596" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/How-much-air-do-people-breathe-in-a-lifetime/-301494"><img src="https://cdn.britannica.com/96/253596-138-3435681B/How-much-air-do-people-breathe-in-a-lifetime.jpg?w=800&h=450&c=crop" alt="How much air do you breathe in a lifetime?" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/t83dZRQL.m3u8" } ], "image": "https://cdn.britannica.com/96/253596-138-3435681B/How-much-air-do-people-breathe-in-a-lifetime.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/EcWmx4cb.vtt", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/Z3ay1o0I.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="253596">How much air do you breathe in a lifetime?</span><span>Every minute, multiple liters of air are cycled through your lungs.</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/oxygen/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">During respiration, animals and some <a href="https://www.britannica.com/science/bacteria" class="md-crosslink autoxref " data-show-preview="true">bacteria</a> take oxygen from the atmosphere and return to it carbon dioxide, whereas by <a href="https://www.britannica.com/science/photosynthesis" class="md-crosslink autoxref " data-show-preview="true">photosynthesis</a>, green plants <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="assimilate" href="https://www.merriam-webster.com/dictionary/assimilate" data-type="MW">assimilate</a> carbon dioxide in the presence of sunlight and evolve free oxygen. Almost all the free oxygen in the atmosphere is due to photosynthesis. About 3 parts of oxygen by volume dissolve in 100 parts of fresh water at 20 °C (68 °F), slightly less in seawater. Dissolved oxygen is essential for the respiration of fish and other marine life.</p><span class="marker MOD5 mod-inline"></span> <span class="marker PREMOD6 mod-inline"></span><p class="topic-paragraph">Natural oxygen is a mixture of three stable isotopes: oxygen-16 (99.759 percent), oxygen-17 (0.037 percent), and oxygen-18 (0.204 percent). Several artificially prepared radioactive isotopes are known. The longest-lived, oxygen-15 (124-second half-life), has been used to study respiration in mammals.</p><span class="marker MOD6 mod-inline"></span> <div class="mb-20"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="299687" data-asm-type="infogram"><div class="md-assembly-wrapper card-media" data-type="infogram"><script id="infogram_0__/z8pqlTWwRHqAnaNwFRkn" src="https://e.infogram.com/js/dist/embed.js?Itp" type="text/javascript" title="Explore an interactive periodic table of the elements"></script></div></figure></div></div> </section> <span class="marker h4"></span><section data-level="1" id="ref279402"> <h2 class="h1">Allotropy</h2> <span class="marker PREMOD8 mod-inline"></span><p class="topic-paragraph">Oxygen has two allotropic forms, diatomic (O<sub>2</sub>) and triatomic (O<sub>3</sub>, ozone). The properties of the diatomic form suggest that six electrons bond the atoms and two electrons remain unpaired, accounting for the <a href="https://www.britannica.com/science/paramagnetism" class="md-crosslink autoxref " data-show-preview="true">paramagnetism</a> of oxygen. The three atoms in the <a href="https://www.britannica.com/science/ozone" class="md-crosslink autoxref " data-show-preview="true">ozone</a> <a href="https://www.britannica.com/science/molecule" class="md-crosslink autoxref " data-show-preview="true">molecule</a> do not lie along a straight line.</p><div class="module-spacing"> <DIV class="marketing-INLINE_SUBSCRIPTION marketing-content" data-marketing-id="INLINE_SUBSCRIPTION"><style> .student-promo-banner-wrapper { container-type: inline-size; margin-bottom: 15px; } @container (min-width: 475px) { .student-promo-banner { flex-direction: row; } .student-promo-banner-img-wrapper { margin-bottom: 0; margin-right: 10px; justify-content: flex-start; } .student-promo-banner-text-wrapper { text-align: left; margin-bottom: 0px; margin-left: 10px; } .student-promo-banner-button-wrapper { margin-right: 0; } }</style> <div class="student-promo-banner-wrapper"> <div class="student-promo-banner d-flex flex-column align-items-center bg-blue rounded p-20"> <div class="student-promo-banner-img-wrapper mb-20 mr-0 d-flex justify-content-center"> <img class="rounded" style="max-width: 100px; min-width: 80px" src="https://cdn.britannica.com/marketing/BlueThistle.webp" /> </div> <div class="student-promo-banner-text-wrapper ml-0 mb-10 text-center text-white"> <div class="h2 mb-10">Get Unlimited Access</div> <div class="h4 font-weight-semi-bold">Try Britannica Premium for free and discover more.</div> </div> <div class="student-promo-banner-button-wrapper d-flex justify-content-center align-items-center ml-auto mr-auto"> <a class="btn btn-m btn-orange" href="https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=inline-cta&utm_campaign=august-2024">Subscribe</a> </div> </div> </div> </DIV></div><span class="marker MOD8 mod-inline"></span> <span class="marker PREMOD9 mod-inline"></span><p class="topic-paragraph">Ozone may be produced from oxygen according to the equation:<span class="md-formula"><span id="ref-16337"></span><img src="https://cdn.britannica.com/37/16337-004-0E2D3EE2/chemical-equation.jpg" alt="Chemical equation." class="inline-image-baseline"></span></p><span class="marker MOD9 mod-inline"></span> <span class="marker PREMOD10 mod-inline"></span><p class="topic-paragraph">The process, as written, is endothermic (energy must be provided to make it proceed); <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="conversion" href="https://www.britannica.com/dictionary/conversion" data-type="EB">conversion</a> of ozone back into diatomic oxygen is promoted by the presence of transition metals or their oxides. Pure oxygen is partly transformed into ozone by a silent electrical discharge; the reaction is also brought about by absorption of <a href="https://www.britannica.com/science/ultraviolet-radiation" class="md-crosslink autoxref " data-show-preview="true">ultraviolet light</a> of wavelengths around 250 nanometres (nm, the nanometre, equal to 10<sup>−9</sup> metre); occurrence of this process in the upper atmosphere removes radiation that would be harmful to life on the surface of the Earth. The pungent odour of ozone is noticeable in confined areas in which there is sparking of electrical equipment, as in generator rooms. Ozone is light blue; its <a href="https://www.britannica.com/science/density" class="md-crosslink autoxref " data-show-preview="true">density</a> is 1.658 times that of air, and it has a <a href="https://www.britannica.com/science/boiling-point" class="md-crosslink autoxref " data-show-preview="true">boiling point</a> of −112 °C (−170 °F) at <a href="https://www.britannica.com/science/atmospheric-pressure" class="md-crosslink autoxref " data-show-preview="true">atmospheric pressure</a>.</p><span class="marker MOD10 mod-inline"></span> <span class="marker PREMOD11 mod-inline"></span><p class="topic-paragraph">Ozone is a powerful oxidizing agent, capable of <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="converting" href="https://www.britannica.com/dictionary/converting" data-type="EB">converting</a> <a href="https://www.britannica.com/science/sulfur-dioxide" class="md-crosslink autoxref " data-show-preview="true">sulfur dioxide</a> to sulfur trioxide, sulfides to sulfates, iodides to <a href="https://www.britannica.com/science/iodine" class="md-crosslink autoxref " data-show-preview="true">iodine</a> (providing an <a href="https://www.britannica.com/science/scientific-method" class="md-crosslink autoxref " data-show-preview="true">analytical method</a> for its estimation), and many organic compounds to oxygenated derivatives such as aldehydes and acids. The conversion by ozone of hydrocarbons from automotive exhaust gases to these acids and aldehydes contributes to the irritating nature of <a href="https://www.britannica.com/science/smog" class="md-crosslink autoxref " data-show-preview="true">smog</a>. Commercially, ozone has been used as a chemical reagent, as a disinfectant, in <a href="https://www.britannica.com/technology/wastewater-treatment" class="md-crosslink autoxref " data-show-preview="true">sewage treatment</a>, <a href="https://www.britannica.com/topic/water-purification" class="md-crosslink autoxref " data-show-preview="true">water purification</a>, and bleaching textiles.</p><span class="marker MOD11 mod-inline"></span> </section> <span class="marker h5"></span><section data-level="1" id="ref279403"> <h2 class="h1">Preparative methods</h2> <span class="marker PREMOD12 mod-inline"></span><p class="topic-paragraph">Production methods chosen for oxygen depend upon the quantity of the element desired. Laboratory procedures include the following:</p><span class="marker MOD12 mod-inline"></span> <span class="marker PREMOD13 mod-inline"></span><p class="topic-paragraph">1. Thermal decomposition of certain salts, such as <a href="https://www.britannica.com/science/potassium" class="md-crosslink autoxref " data-show-preview="true">potassium</a> chlorate or potassium nitrate:<span class="md-formula"><span id="ref-16336"></span><img src="https://cdn.britannica.com/36/16336-004-7B82A073/Chemical-equations.jpg" alt="Chemical equations." class="inline-image-baseline"></span></p><span class="marker MOD13 mod-inline"></span> <span class="marker PREMOD14 mod-inline"></span><p class="topic-paragraph">The decomposition of potassium chlorate is catalyzed by oxides of transition metals; <a href="https://www.britannica.com/science/manganese" class="md-crosslink autoxref " data-show-preview="true">manganese</a> dioxide (pyrolusite, MnO<sub>2</sub>) is frequently used. The temperature necessary to effect the evolution of oxygen is reduced from 400 °C to 250 °C by the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="catalyst" href="https://www.merriam-webster.com/dictionary/catalyst" data-type="MW">catalyst</a>.</p><span class="marker MOD14 mod-inline"></span> <span class="marker PREMOD15 mod-inline"></span><p class="topic-paragraph">2. Thermal decomposition of oxides of heavy metals:<span class="md-formula"><span id="ref-16335"></span><img src="https://cdn.britannica.com/35/16335-004-74E3579C/Chemical-equations.jpg" alt="Chemical equations." class="inline-image-baseline"></span></p><span class="marker MOD15 mod-inline"></span> <span class="marker PREMOD16 mod-inline"></span><p class="topic-paragraph">Scheele and Priestley used mercury(II) oxide in their preparations of oxygen.</p><span class="marker MOD16 mod-inline"></span> <span class="marker PREMOD17 mod-inline"></span><p class="topic-paragraph">3. Thermal decomposition of metal peroxides or of <a href="https://www.britannica.com/science/hydrogen" class="md-crosslink autoxref " data-show-preview="true">hydrogen</a> peroxide:<span class="md-formula"><span id="ref-16334"></span><img src="https://cdn.britannica.com/34/16334-004-93565CAC/Chemical-equations.jpg" alt="Chemical equations." class="inline-image-baseline"></span></p><span class="marker MOD17 mod-inline"></span> <span class="marker PREMOD18 mod-inline"></span><p class="topic-paragraph">An early commercial procedure for isolating oxygen from the atmosphere or for manufacture of <a href="https://www.britannica.com/science/hydrogen-peroxide" class="md-crosslink autoxref " data-show-preview="true">hydrogen peroxide</a> depended on the formation of <a href="https://www.britannica.com/science/barium" class="md-crosslink autoxref " data-show-preview="true">barium</a> <a href="https://www.britannica.com/science/peroxide" class="md-crosslink autoxref " data-show-preview="true">peroxide</a> from the oxide as shown in the equations.</p><span class="marker MOD18 mod-inline"></span> <span class="marker PREMOD19 mod-inline"></span><p class="topic-paragraph">4. Electrolysis of water containing small proportions of salts or acids to allow conduction of the electric current:<span class="md-formula"><span id="ref-16333"></span><img src="https://cdn.britannica.com/33/16333-004-5C128EAC/chemical-equation.jpg" alt="Chemical equation." class="inline-image-baseline"></span></p><span class="marker MOD19 mod-inline"></span> </section> <span class="marker h6"></span><section data-level="1" id="ref279404"> <h2 class="h1">Commercial production and use</h2> <span class="marker PREMOD20 mod-inline"></span><p class="topic-paragraph">When required in tonnage quantities, oxygen is prepared by the fractional <a href="https://www.britannica.com/science/distillation" class="md-crosslink autoxref " data-show-preview="true">distillation</a> of liquid air. Of the main components of air, oxygen has the highest boiling point and therefore is less <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="volatile" href="https://www.britannica.com/dictionary/volatile" data-type="EB">volatile</a> than <a href="https://www.britannica.com/science/nitrogen" class="md-crosslink autoxref " data-show-preview="true">nitrogen</a> and <a href="https://www.britannica.com/science/argon-chemical-element" class="md-crosslink autoxref " data-show-preview="true">argon</a>. The process takes advantage of the fact that when a compressed gas is allowed to expand, it cools. Major steps in the operation include the following: (1) Air is filtered to remove particulates; (2) moisture and carbon dioxide are removed by absorption in alkali; (3) the air is compressed and the heat of compression removed by ordinary cooling procedures; (4) the compressed and cooled air is passed into coils contained in a chamber; (5) a portion of the <a href="https://www.britannica.com/science/compressed-air" class="md-crosslink autoxref " data-show-preview="true">compressed air</a> (at about 200 atmospheres pressure) is allowed to expand in the chamber, cooling the coils; (6) the expanded gas is returned to the compressor with multiple subsequent expansion and compression steps resulting finally in liquefaction of the compressed air at a temperature of −196 °C; (7) the liquid air is allowed to warm to distill first the light rare gases, then the nitrogen, leaving liquid oxygen. Multiple fractionations will produce a product pure enough (99.5 percent) for most industrial purposes.</p><span class="marker MOD20 mod-inline"></span> <span class="marker PREMOD21 mod-inline"></span><p class="topic-paragraph">The <a href="https://www.britannica.com/technology/steel" class="md-crosslink autoxref " data-show-preview="true">steel</a> industry is the largest consumer of pure oxygen in “blowing” high carbon steel—that is, volatilizing carbon dioxide and other <a href="https://www.britannica.com/science/nonmetal" class="md-crosslink autoxref " data-show-preview="true">nonmetal</a> impurities in a more rapid and more easily controlled process than if air were used. The treatment of sewage by oxygen holds promise for more efficient treatment of liquid effluents than other chemical processes. Incineration of wastes in closed systems using pure oxygen has become important. The so-called LOX of <a href="https://www.britannica.com/technology/rocket-jet-propulsion-device-and-vehicle" class="md-crosslink autoxref " data-show-preview="true">rocket</a> oxidizer fuels is liquid oxygen; the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="consumption" href="https://www.merriam-webster.com/dictionary/consumption" data-type="MW">consumption</a> of LOX depends upon the activity of space programs. Pure oxygen is used in submarines and diving bells.</p><span class="marker MOD21 mod-inline"></span> <span class="marker PREMOD22 mod-inline"></span><p class="topic-paragraph">Commercial oxygen or oxygen-enriched air has replaced ordinary air in the <a href="https://www.britannica.com/technology/chemical-industry" class="md-crosslink autoxref " data-show-preview="true">chemical industry</a> for the manufacture of such oxidation-controlled chemicals as <a href="https://www.britannica.com/science/acetylene" class="md-crosslink autoxref " data-show-preview="true">acetylene</a>, <a href="https://www.britannica.com/science/ethylene" class="md-crosslink autoxref " data-show-preview="true">ethylene</a> oxide, and <a href="https://www.britannica.com/science/methanol" class="md-crosslink autoxref " data-show-preview="true">methanol</a>. Medical applications of oxygen include use in oxygen tents, inhalators, and pediatric incubators. Oxygen-enriched gaseous anesthetics ensure life support during general anesthesia. Oxygen is significant in a number of industries that use kilns.</p><span class="marker MOD22 mod-inline"></span> </section> <span class="marker h7"></span><section data-level="1" id="ref279405"> <h2 class="h1">Chemical properties and reactions</h2> <span class="marker PREMOD23 mod-inline"></span><p class="topic-paragraph">The large values of the <a href="https://www.britannica.com/science/electronegativity" class="md-crosslink autoxref " data-show-preview="true">electronegativity</a> and the <a href="https://www.britannica.com/science/electron-affinity" class="md-crosslink autoxref " data-show-preview="true">electron affinity</a> of oxygen are typical of elements that show only nonmetallic behaviour. In all of its compounds, oxygen assumes a negative <a href="https://www.britannica.com/science/oxidation-number" class="md-crosslink autoxref " data-show-preview="true">oxidation state</a> as is expected from the two half-filled outer orbitals. When these orbitals are filled by electron transfer, the oxide ion O<sup>2−</sup> is created. In peroxides (species containing the ion O<sub>2</sub><sup>2−</sup>) it is assumed that each oxygen has a charge of −1. This property of accepting electrons by complete or partial transfer defines an oxidizing agent. When such an agent reacts with an electron-donating substance, its own oxidation state is lowered. The change (lowering), from the zero to the −2 state in the case of oxygen, is called a reduction. Oxygen may be thought of as the “original” oxidizing agent, the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="nomenclature" href="https://www.merriam-webster.com/dictionary/nomenclature" data-type="MW">nomenclature</a> used to describe oxidation and reduction being based upon this behaviour typical of oxygen.</p><span class="marker MOD23 mod-inline"></span> <span class="marker PREMOD24 mod-inline"></span><p class="topic-paragraph">As described in the section on <a href="https://www.britannica.com/science/allotropy" class="md-crosslink autoxref " data-show-preview="true">allotropy</a>, oxygen forms the diatomic species, O<sub>2</sub>, under normal conditions and, as well, the triatomic species ozone, O<sub>3</sub>. There is some evidence for a very unstable tetratomic species, O<sub>4</sub>. In the molecular diatomic form there are two unpaired electrons that lie in antibonding orbitals. The paramagnetic behaviour of oxygen confirms the presence of such electrons.</p><span class="marker MOD24 mod-inline"></span> <span class="marker PREMOD25 mod-inline"></span><p class="topic-paragraph">The intense reactivity of ozone is sometimes explained by suggesting that one of the three oxygen atoms is in an “atomic” state; on reacting, this atom is dissociated from the O<sub>3</sub> molecule, leaving molecular oxygen.</p><span class="marker MOD25 mod-inline"></span> <span class="marker PREMOD26 mod-inline"></span><p class="topic-paragraph">The molecular species, O<sub>2</sub>, is not especially reactive at normal (ambient) temperatures and pressures. The atomic species, O, is far more reactive. The energy of dissociation (O<sub>2</sub> → 2O) is large at 117.2 kilocalories per mole.</p><div class="one-good-fact-module"> </div><span class="marker MOD26 mod-inline"></span> <span class="marker PREMOD27 mod-inline"></span><p class="topic-paragraph">Oxygen has an oxidation state of −2 in most of its compounds. It forms a large range of covalently bonded compounds, among which are oxides of nonmetals, such as water (H<sub>2</sub>O), sulfur dioxide (SO<sub>2</sub>), and carbon dioxide (CO<sub>2</sub>); organic compounds such as alcohols, aldehydes, and carboxylic acids; common acids such as sulfuric (H<sub>2</sub>SO<sub>4</sub>), carbonic (H<sub>2</sub>CO<sub>3</sub>), and nitric (HNO<sub>3</sub>); and corresponding salts, such as <a href="https://www.britannica.com/science/sodium" class="md-crosslink autoxref " data-show-preview="true">sodium</a> sulfate (Na<sub>2</sub>SO<sub>4</sub>), sodium carbonate (Na<sub>2</sub>CO<sub>3</sub>), and <a href="https://www.britannica.com/science/Chile-saltpetre" class="md-crosslink autoxref " data-show-preview="true">sodium nitrate</a> (NaNO<sub>3</sub>). Oxygen is present as the oxide <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="ion" href="https://www.britannica.com/dictionary/ion" data-type="EB">ion</a>, O<sup>2</sup><sup>-</sup>, in the crystalline structure of solid metallic oxides such as <a href="https://www.britannica.com/science/quicklime" class="md-crosslink autoxref " data-show-preview="true">calcium oxide</a>, CaO. Metallic superoxides, such as potassium superoxide, KO<sub>2</sub>, contain the O<sub>2</sub><sup>-</sup> ion, whereas metallic peroxides, such as barium peroxide, BaO<sub>2</sub>, contain the O<sub>2</sub><sup>2-</sup> ion.</p><span class="marker MOD27 mod-inline"></span> <span class="md-signature font-12"><a href="/contributor/Robert-C-Brasted/360">Robert C. 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Oxygen | Discovery, Symbol, Properties, Uses, & Facts | Britannica