Hydrogen | Properties, Uses, & Facts

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border-right-sm border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 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/hydrogen">hydrogen</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/hydrogen">Introduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref80844"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/hydrogen#ref80844">Physical and chemical properties</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref80845"><a class="w-100 link-gray-900" href="/science/hydrogen#ref80845">Ortho-hydrogen and para-hydrogen</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref80846"><a class="w-100 link-gray-900" href="/science/hydrogen/Reactivity-of-hydrogen">Reactivity of hydrogen</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref80847"><a class="w-100 link-gray-900" href="/science/hydrogen/Reactivity-of-hydrogen#ref80847">Hydrogen bond</a></li></ul></div></li><li data-target="#ref80848"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/hydrogen/Isotopes-of-hydrogen">Isotopes of hydrogen</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref80849"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/hydrogen/Production-and-applications-of-hydrogen">Production and applications of hydrogen</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref80850"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/hydrogen/Production-and-applications-of-hydrogen#ref80850">Analysis</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/hydrogen/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/hydrogen">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/hydrogen/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/99/22399-050-FFE3FA25/Hydrogen-H-Its-symbol-square-hydrogen-properties.jpg" data-href="/media/1/278523/64466" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/99/22399-004-1AB3EFAB/Hydrogen-H-Its-symbol-square-hydrogen-properties.jpg" alt="chemical properties of hydrogen" height="50" /> </a> <a href="" data-href="/media/1/278523/299687" 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="/video/Catalyst-splits-water-into-hydrogen-and-oxygen/246835" data-href="/media/1/278523/246835" 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/34/216534-138-645B4F31/Catalyst-splits-water-into-hydrogen-and-oxygen.jpg?w=400&h=225&c=crop" alt="Learn about new water molecule-splitting technology that separates hydrogen and oxygen" 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="https://cdn.britannica.com/14/5714-004-44B1517F/Balmer-series-hydrogen-spectrometer.jpg" data-href="/media/1/278523/3155" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/14/5714-004-44B1517F/Balmer-series-hydrogen-spectrometer.jpg" alt="Balmer series of hydrogen" height="50" /> </a> <a href="https://cdn.britannica.com/65/7465-004-4ABF79AE/Explosion-limits-hydrogen-oxygen-mixture.jpg" data-href="/media/1/278523/925" 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/38/6038-050-DC1FDAEA/ionization-energy-element-atom-electron-energies-nonmetal.jpg" data-href="/media/1/278523/647" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/38/6038-004-134735D2/ionization-energy-element-atom-electron-energies-nonmetal.jpg" alt="first ionization energies of the elements" height="50" /> </a> <a href="https://cdn.britannica.com/36/6036-050-EB45F0EA/Energy-levels-hydrogen-atom-model-quantum-mechanics.jpg" data-href="/media/1/278523/645" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/36/6036-004-7FC6A797/Energy-levels-hydrogen-atom-model-quantum-mechanics.jpg" alt="energy levels of the hydrogen atom" height="50" /> </a> <a href="https://cdn.britannica.com/28/6028-004-2E070A83/Transformations-hydrogen-compounds-atmosphere.jpg" data-href="/media/1/278523/636" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/28/6028-004-2E070A83/Transformations-hydrogen-compounds-atmosphere.jpg" alt="Figure 15: Transformations of hydrogen compounds in the atmosphere." height="50" /> </a> <a href="https://cdn.britannica.com/37/6037-050-A39F57B7/Electron-densities-wave-functions-equation-Schrodinger-lowest.jpg" data-href="/media/1/278523/646" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/37/6037-004-DCBBDBE4/Electron-densities-wave-functions-equation-Schrodinger-lowest.jpg" alt="Electron densities in wave functions of the Schrödinger equation" height="50" /> </a> <a href="https://cdn.britannica.com/83/5683-004-1BA01A0E/hydrogen-atom-states-Electron-density-functions-Plots.jpg" data-href="/media/1/278523/3149" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/83/5683-004-1BA01A0E/hydrogen-atom-states-Electron-density-functions-Plots.jpg" alt="electron density functions for four hydrogen atom states" height="50" /> </a> </div> <button disabled class="prev-button js-prev-button position-absolute btn btn-circle shadow btn-blue " aria-label="Previous"> <span 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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="2025-01-26T00:00:00CST" >Jan 26, 2025</time> •</span> <a class="byline-edit-history" href="https://www.britannica.com/science/hydrogen/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-Cavendish" topicid="100664">Henry Cavendish</a></dd> <dd><a href="/biography/Antoine-Lavoisier" topicid="332700">Antoine Lavoisier</a></dd> <dd><a href="/biography/Otto-Struve" topicid="569744">Otto Struve</a></dd> <dd><a href="/biography/Anders-Jonas-Angstrom" topicid="25262">Anders Jonas Ångström</a></dd> <dd><a href="/biography/Fritz-Wolfgang-London" topicid="346938">Fritz Wolfgang London</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/hydride" topicid="278278">hydride</a></dd> <dd><a href="/science/deuterium" topicid="159684">deuterium</a></dd> <dd><a href="/science/air" topicid="10582">air</a></dd> <dd><a href="/science/tritium" topicid="606002">tritium</a></dd> <dd><a href="/science/protium-isotope" topicid="480103">protium</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/hydrogen"> See all related content </a> </div> </div> </div> </div><div class="bg-gray-50 p-15 rounded module-spacing recent-news d-flex flex-column float-false"> <div> <h2 class="font-weight-bold font-14 m-0 d-inline"> News <span class="text-gray-600">•</span> </h2> <div class="recent-news-item first-recent-news-item d-inline"> <a class="font-14 gtm-ap-news-link" href="https://techxplore.com/news/2025-01-hydrogen-minutes-microwaves-energy-faster.html" rel="nofollow">Clean hydrogen in minutes: Microwaves deliver clean energy faster</a> <span class="font-14 text-gray-600"> <span>•</span> Jan. 21, 2025, 4:40 AM ET (Tech Xplore) </span> </div> </div> <div class="rest-of-recent-news-items"> <button class="js-toggle-recent-news d-flex btn btn-unstyled font-14 pr-10 rounded-sm mt-10" aria-label="Toggle additional news items"> Show less <em class="material-icons" data-icon="expand_less"></em> </button> </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="ref621337"></span>hydrogen (H)</strong>, a colourless, odourless, tasteless, flammable gaseous substance that is the simplest member of the family of chemical elements. The hydrogen <a href="https://www.britannica.com/science/atom" class="md-crosslink autoxref " data-show-preview="true">atom</a> has a nucleus consisting of a <a href="https://www.britannica.com/science/proton-subatomic-particle" class="md-crosslink autoxref " data-show-preview="true">proton</a> <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="bearing" href="https://www.britannica.com/dictionary/bearing" data-type="EB">bearing</a> one unit of positive electrical charge; an electron, bearing one unit of negative <a href="https://www.britannica.com/science/electric-charge" class="md-crosslink autoxref " data-show-preview="true">electrical charge</a>, is also associated with this nucleus. Under ordinary conditions, hydrogen <a href="https://www.britannica.com/science/gas-state-of-matter" class="md-crosslink autoxref " data-show-preview="true">gas</a> is a loose aggregation of hydrogen molecules, each consisting of a pair of atoms, a <a href="https://www.britannica.com/science/diatomic-molecule" class="md-crosslink autoxref " data-show-preview="true">diatomic molecule</a>, H<sub>2</sub>. The earliest known important chemical property of hydrogen is that it burns with <a href="https://www.britannica.com/science/oxygen" class="md-crosslink autoxref " data-show-preview="true">oxygen</a> to form <a href="https://www.britannica.com/science/water" class="md-crosslink autoxref " data-show-preview="true">water</a>, H<sub>2</sub>O; indeed, the name hydrogen is derived from Greek words meaning “maker of water.”</p><span class="marker MOD1 mod-inline"></span><span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">Although hydrogen is the most abundant element in the <a href="https://www.britannica.com/science/universe" class="md-crosslink autoxref " data-show-preview="true">universe</a> (three times as abundant as <a href="https://www.britannica.com/science/helium-chemical-element" class="md-crosslink autoxref " data-show-preview="true">helium</a>, the next most widely occurring element), it makes up only about 0.14 percent of Earth’s crust by weight. It occurs, however, in vast quantities as part of the water in oceans, <a href="https://www.britannica.com/science/ice" class="md-crosslink autoxref " data-show-preview="true">ice</a> packs, rivers, lakes, and the atmosphere. As part of innumerable <a href="https://www.britannica.com/science/carbon-chemical-element" class="md-crosslink autoxref " data-show-preview="true">carbon</a> <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>, hydrogen is present in all animal and vegetable tissue and in petroleum. Even though it is often said that there are more known compounds of carbon than of any other element, the fact is that, since hydrogen is contained in almost all carbon compounds and also forms a multitude of compounds with all other elements (except some of the noble gases), it is possible that hydrogen compounds are more numerous.</p><span class="marker MOD2 mod-inline"></span><span class="marker PREMOD3 mod-inline"></span><p class="topic-paragraph">Elementary hydrogen finds its principal industrial application in the manufacture of <a href="https://www.britannica.com/science/ammonia" class="md-crosslink autoxref " data-show-preview="true">ammonia</a> (a <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> of hydrogen and <a href="https://www.britannica.com/science/nitrogen" class="md-crosslink autoxref " data-show-preview="true">nitrogen</a>, NH<sub>3</sub>) and in the <a href="https://www.britannica.com/science/hydrogenation" class="md-crosslink autoxref " data-show-preview="true">hydrogenation</a> of <a href="https://www.britannica.com/science/carbon-monoxide" class="md-crosslink autoxref " data-show-preview="true">carbon monoxide</a> and organic compounds.</p><span class="marker MOD3 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><span class="marker PREMOD5 mod-inline"></span><p class="topic-paragraph">Hydrogen has three known isotopes. The mass numbers of hydrogen’s isotopes are 1, 2, and 3, the most abundant being the mass 1 <a href="https://www.britannica.com/science/isotope" class="md-crosslink autoxref " data-show-preview="true">isotope</a> generally called hydrogen (symbol H, or <sup>1</sup>H) but also known as <span id="ref621338"></span><a href="https://www.britannica.com/science/protium-isotope" class="md-crosslink " data-show-preview="true">protium</a>. The mass 2 isotope, which has a nucleus of one proton and one <a href="https://www.britannica.com/science/neutron" class="md-crosslink autoxref " data-show-preview="true">neutron</a> and has been named <a href="https://www.britannica.com/science/deuterium" class="md-crosslink autoxref " data-show-preview="true">deuterium</a>, or heavy hydrogen (symbol D, or <sup>2</sup>H), <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="constitutes" href="https://www.merriam-webster.com/dictionary/constitutes" data-type="MW">constitutes</a> 0.0156 percent of the ordinary mixture of hydrogen. Tritium (symbol T, or <sup>3</sup>H), with one proton and two neutrons in each nucleus, is the mass 3 isotope and constitutes about 10<sup>−15</sup> to 10<sup>−16</sup> percent of hydrogen. The practice of giving distinct names to the hydrogen isotopes is justified by the fact that there are significant differences in their properties.</p><a class="link-module shadow-sm d-block qa-quiz-module" href="/quiz/facts-you-should-know-the-periodic-table-quiz" data-link-module-iframe-link=""> <img loading="lazy" src="https://cdn.britannica.com/58/203458-131-D94E9327/periodic-table-concept.jpg" alt="Periodic Table of the elements concept image (chemistry)" 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">Facts You Should Know: The Periodic Table Quiz</div> </div> </a><span class="marker MOD5 mod-inline"></span><span class="marker PREMOD6 mod-inline"></span><p class="topic-paragraph">Paracelsus, physician and alchemist, in the 16th century unknowingly experimented with hydrogen when he found that a flammable gas was evolved when a <a href="https://www.britannica.com/science/metal-chemistry" class="md-crosslink autoxref " data-show-preview="true">metal</a> was dissolved in <a href="https://www.britannica.com/science/acid" class="md-crosslink autoxref " data-show-preview="true">acid</a>. The gas, however, was confused with other flammable gases, such as hydrocarbons and carbon monoxide. In 1766 <span id="ref621339"></span><a href="https://www.britannica.com/biography/Henry-Cavendish" class="md-crosslink " data-show-preview="true">Henry Cavendish</a>, English chemist and physicist, showed that hydrogen, then called flammable <a href="https://www.britannica.com/science/air" class="md-crosslink autoxref " data-show-preview="true">air</a>, <a href="https://www.britannica.com/science/phlogiston" class="md-crosslink autoxref " data-show-preview="true">phlogiston</a>, or the flammable principle, was distinct from other combustible gases because of its <a href="https://www.britannica.com/science/density" class="md-crosslink autoxref " data-show-preview="true">density</a> and the amount of it that evolved from a given amount of acid and metal. In 1781 Cavendish confirmed previous observations that water was formed when hydrogen was burned, and <a href="https://www.britannica.com/biography/Antoine-Lavoisier" class="md-crosslink autoxref " data-show-preview="true">Antoine-Laurent Lavoisier</a>, the father of modern <a href="https://www.britannica.com/science/chemistry" class="md-crosslink autoxref " data-show-preview="true">chemistry</a>, coined the French word <em>hydrogène</em> from which the English form is <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="derived" href="https://www.britannica.com/dictionary/derived" data-type="EB">derived</a>. In 1929 Karl Friedrich Bonhoeffer, a German physical chemist, and Paul Harteck, an Austrian chemist, on the basis of earlier theoretical work, showed that ordinary hydrogen is a mixture of two kinds of molecules, <em>ortho</em>-hydrogen and <em>para</em>-hydrogen. Because of the simple structure of hydrogen, its properties can be theoretically calculated relatively easily. Hence hydrogen is often used as a theoretical model for more complex atoms, and the results are applied qualitatively to other atoms.</p><span class="marker MOD6 mod-inline"></span></section> <span class="marker h2"></span><section data-level="1" id="ref80844"> <h2 class="h1">Physical and chemical properties</h2> <span class="marker PREMOD7 mod-inline"></span><p class="topic-paragraph">The <span id="ref2642"></span>Table lists the important properties of molecular hydrogen, H<sub>2</sub>. The extremely low melting and boiling points result from weak forces of attraction between the molecules. The existence of these weak intermolecular forces is also revealed by the fact that, when hydrogen gas expands from high to <a href="https://www.britannica.com/science/cyclone-meteorology" class="md-crosslink autoxref " data-show-preview="true">low pressure</a> at room temperature, its temperature rises, whereas the temperature of most other gases falls. According to thermodynamic principles, this implies that repulsive forces <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="exceed" href="https://www.britannica.com/dictionary/exceed" data-type="EB">exceed</a> attractive forces between hydrogen molecules at room temperature—otherwise, the expansion would cool the hydrogen. In fact, at −68.6° C attractive forces predominate, and hydrogen, therefore, cools upon being allowed to expand below that temperature. The cooling effect becomes so pronounced at temperatures below that of liquid nitrogen (−196° C) that the effect is utilized to achieve the liquefaction temperature of hydrogen gas itself.</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 MOD7 mod-inline"></span><span class="md-raw-html"><div class="md-drag md-table-wrapper"><table> <caption> Some properties of normal hydrogen and deuterium </caption> <thead> <tr> <th id="1" scope="col"></th> <th id="n" scope="col"> normal hydrogen </th> <th id="d" scope="col"> deuterium </th> </tr> </thead> <thead> <tr> <th colspan="3" id="a" scope="col"> Atomic hydrogen </th> </tr> </thead> <tbody> <tr> <td headers="a" scope="row"> atomic number </td> <td headers="a n"> 1 </td> <td headers="a d"> 1 </td> </tr> <tr> <td headers="a" scope="row"> atomic weight </td> <td headers="a n"> 1.0080 </td> <td headers="a d"> 2.0141 </td> </tr> <tr> <td headers="a" scope="row"> ionization potential </td> <td headers="a n"> 13.595 electron volts </td> <td headers="a d"> 13.600 electron volts </td> </tr> <tr> <td headers="a" scope="row"> electron affinity </td> <td headers="a n"> 0.7542 electron volts </td> <td headers="a d"> 0.754 electron volts </td> </tr> <tr> <td headers="a" scope="row"> nuclear spin </td> <td headers="a n"> 1/2 </td> <td headers="a d"> 1 </td> </tr> <tr> <td headers="a" scope="row"> nuclear magnetic moment (nuclear magnetons) </td> <td headers="a n"> 2.7927 </td> <td headers="a d"> 0.8574 </td> </tr> <tr> <td headers="a" scope="row"> nuclear quadrupole moment </td> <td headers="a n"> 0 </td> <td headers="a d"> 2.77(10<sup>−27</sup>) square centimetres </td> </tr> <tr> <td headers="a" scope="row"> electronegativity (Pauling) </td> <td headers="a n"> 2.1 </td> <td headers="a d"> ~2.1 </td> </tr> </tbody> <thead> <tr> <th colspan="3" id="m" scope="col"> Molecular hydrogen </th> </tr> </thead> <tbody> <tr> <td headers="m" scope="row"> bond distance </td> <td headers="a n"> 0.7416 angstrom </td> <td headers="a n"> 0.7416 angstrom </td> </tr> <tr> <td headers="m" scope="row"> dissociation energy (25 degrees C) </td> <td headers="m n"> 104.19 kilocalories per mole </td> <td headers="m d"> 105.97 kilocalories per mole </td> </tr> <tr> <td headers="m" scope="row"> ionization potential </td> <td headers="m n"> 15.427 electron volts </td> <td headers="m d"> 15.457 electron volts </td> </tr> <tr> <td headers="m" scope="row"> density of solid </td> <td headers="m n"> 0.08671 gram per cubic centimetre </td> <td headers="m d"> 0.1967 gram per cubic centimetre </td> </tr> <tr> <td headers="m" scope="row"> melting point </td> <td headers="m n"> −259.20 degrees Celsius </td> <td headers="m d"> −254.43 degrees Celsius </td> </tr> <tr> <td headers="m" scope="row"> heat of fusion </td> <td headers="m n"> 28 calories per mole </td> <td headers="m d"> 47 calories per mole </td> </tr> <tr> <td headers="m" scope="row"> density of liquid </td> <td headers="m n"> 0.07099 (−252.78 degrees) </td> <td headers="m d"> 0.1630 (−249.75 degrees) </td> </tr> <tr> <td headers="m" scope="row"> boiling point </td> <td headers="m n"> −252.77 degrees Celsius </td> <td headers="m d"> −249.49 degrees Celsius </td> </tr> <tr> <td headers="m" scope="row"> heat of vaporization </td> <td headers="m n"> 216 calories per mole </td> <td headers="m d"> 293 calories per mole </td> </tr> <tr> <td headers="m" scope="row"> critical temperature </td> <td headers="m n"> −240.0 degrees Celsius </td> <td headers="m d"> −243.8 degrees Celsius </td> </tr> <tr> <td headers="m" scope="row"> critical pressure </td> <td headers="m n"> 13.0 atmospheres </td> <td headers="m d"> 16.4 atmospheres </td> </tr> <tr> <td headers="m" scope="row"> critical density </td> <td headers="m n"> 0.0310 gram per cubic centimetre </td> <td headers="m d"> 0.0668 gram per cubic centimetre </td> </tr> <tr> <td headers="m" scope="row"> heat of combustion to water (g) </td> <td headers="m n"> −57.796 kilocalories per mole </td> <td headers="m d"> −59.564 kilocalories per mole </td> </tr> </tbody> </table></div> </span> <span class="marker PREMOD8 mod-inline"></span><p class="topic-paragraph">Hydrogen is transparent to visible light, to infrared light, and to <a href="https://www.britannica.com/science/ultraviolet-radiation" class="md-crosslink autoxref " data-show-preview="true">ultraviolet light</a> to wavelengths below 1800 Å. Because its <a href="https://www.britannica.com/science/molecular-weight" class="md-crosslink autoxref " data-show-preview="true">molecular weight</a> is lower than that of any other gas, its molecules have a velocity higher than those of any other gas at a given temperature and it <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="diffuses" href="https://www.britannica.com/dictionary/diffuses" data-type="EB">diffuses</a> faster than any other gas. Consequently, <a href="https://www.britannica.com/science/kinetic-energy" class="md-crosslink autoxref " data-show-preview="true">kinetic energy</a> is distributed faster through hydrogen than through any other gas; it has, for example, the greatest heat conductivity.</p><span class="marker MOD8 mod-inline"></span> <span class="marker PREMOD9 mod-inline"></span><p class="topic-paragraph">A <span id="ref621340"></span><a href="https://www.britannica.com/science/molecule" class="md-crosslink " data-show-preview="true">molecule</a> of hydrogen is the simplest possible molecule. It consists of two protons and two electrons held together by electrostatic forces. Like atomic hydrogen, the assemblage can exist in a number of energy levels.</p><span class="marker MOD9 mod-inline"></span> <section data-level="2" id="ref80845"> <h2 class="h2"><span id="ref621341"></span>Ortho-hydrogen and <span id="ref621342"></span>para-hydrogen</h2> <span class="marker PREMOD10 mod-inline"></span><p class="topic-paragraph">Two types of molecular hydrogen (<em>ortho</em> and <em>para</em>) are known. These differ in the magnetic <span id="ref621343"></span><a href="https://www.britannica.com/science/isomerism" class="md-crosslink " data-show-preview="true">interactions</a> of the <span id="ref621345"></span><a href="https://www.britannica.com/science/proton-subatomic-particle" class="md-crosslink " data-show-preview="true">protons</a> due to the <span id="ref621344"></span><a href="https://www.britannica.com/science/spin-atomic-physics" class="md-crosslink " data-show-preview="true">spinning</a> motions of the protons. In <em>ortho</em>-hydrogen, the spins of both protons are aligned in the same direction—that is, they are parallel. In <em>para</em>-hydrogen, the spins are aligned in opposite directions and are therefore antiparallel. The relationship of <a href="https://www.britannica.com/science/spin-atomic-physics" class="md-crosslink autoxref " data-show-preview="true">spin</a> alignments determines the magnetic properties of the <span id="ref621346"></span><a href="https://www.britannica.com/science/atomic-nucleus" class="md-crosslink ">atoms</a>. Normally, transformations of one type into the other (<em>i.e.,</em> conversions between <em>ortho</em> and <em>para</em> molecules) do not occur and <em>ortho</em>-hydrogen and <em>para</em>-hydrogen can be regarded as two distinct modifications of hydrogen. The two forms may, however, interconvert under certain conditions. <span id="ref621347"></span><a href="https://www.britannica.com/science/thermodynamic-equilibrium" class="md-crosslink " data-show-preview="true">Equilibrium</a> between the two forms can be established in several ways. One of these is by the introduction of <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="catalysts" href="https://www.merriam-webster.com/dictionary/catalysts" data-type="MW">catalysts</a> (such as activated charcoal or various paramagnetic substances); another method is to apply an electrical discharge to the gas or to heat it to a high temperature.</p><span class="marker MOD10 mod-inline"></span> <span class="marker PREMOD11 mod-inline"></span><p class="topic-paragraph">The concentration of <em>para</em>-hydrogen in a mixture that has achieved <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="equilibrium" href="https://www.merriam-webster.com/dictionary/equilibrium" data-type="MW">equilibrium</a> between the two forms depends on the temperature as shown by the following figures:</p><span class="marker MOD11 mod-inline"></span> <span class="marker PREMOD12 mod-inline"></span><p class="topic-paragraph"><span id="ref-16360"></span><img src="https://cdn.britannica.com/60/16360-004-03C6F013/List-concentrations-para-hydrogen-temperatures.jpg" alt="List of concentrations of para-hydrogen at various temperatures." class="inline-image-baseline"></p><span class="marker MOD12 mod-inline"></span> <span class="marker PREMOD13 mod-inline"></span><p class="topic-paragraph">Essentially pure <em>para</em>-hydrogen can be produced by bringing the mixture into contact with charcoal at the temperature of liquid hydrogen; this converts all the <em>ortho</em>-hydrogen into <em>para</em>-hydrogen. The <em>ortho</em>-hydrogen, on the other hand, cannot be prepared directly from the mixture because the concentration of <em>para</em>-hydrogen is never less than 25 percent.</p><div class="one-good-fact-module"> </div><span class="marker MOD13 mod-inline"></span> <span class="marker PREMOD14 mod-inline"></span><p class="topic-paragraph">The two forms of hydrogen have slightly different physical properties. The <a href="https://www.britannica.com/science/melting-point" class="md-crosslink autoxref " data-show-preview="true">melting point</a> of <em>para</em>-hydrogen is 0.10° lower than that of a 3:1 mixture of <em>ortho</em>-hydrogen and <em>para</em>-hydrogen. At −252.77° C the pressure exerted by the vapour over liquid <em>para</em>-hydrogen is 1.035 atmospheres (one atmosphere is the pressure of the atmosphere at <a href="https://www.britannica.com/science/sea-level" class="md-crosslink autoxref " data-show-preview="true">sea level</a> under standard conditions, equal to about 14.69 pounds per square inch), compared with 1.000 atmosphere for the <a href="https://www.britannica.com/science/vapor-pressure" class="md-crosslink autoxref " data-show-preview="true">vapour pressure</a> of the 3:1 <em>ortho–para</em> mixture. As a result of the different vapour pressures of <em>para</em>-hydrogen and <em>ortho</em>-hydrogen, these forms of hydrogen can be separated by low-temperature <a href="https://www.britannica.com/science/gas-chromatography" class="md-crosslink autoxref " data-show-preview="true">gas chromatography</a>, an <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="analytical" href="https://www.merriam-webster.com/dictionary/analytical" data-type="MW">analytical</a> process that separates different atomic and molecular species on the basis of their differing volatilities.</p><span class="marker MOD14 mod-inline"></span> </section> </section><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":4,"pageId":278523,"pageLength":1218,"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-page.js?v=3.133.9' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"A","adLeg":"A","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"PAGINATED","gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":true} </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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Hydrogen | Properties, Uses, & Facts | Britannica