Editing Period 1 element

{{Short description|Element in first row of periodic table}}
{{Redirect|Period 1|similarly named topics|First period (disambiguation)}}
{{Periodic table (micro)| title=Period 1 in the [[periodic table]] |  mark=H,He}}
{{Sidebar periodic table|expanded=structure }}
A '''period&nbsp;1 element''' is one of the [[chemical element]]s in the first row (or [[Periodic table period|period]]) of the [[periodic table|periodic table of the chemical elements]]. The periodic table is laid out in rows to illustrate periodic (recurring) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that analog elements fall into the same vertical columns. The first period contains fewer elements than any other row in the table, with only two: [[hydrogen]] and [[helium]]. This situation can be explained by modern theories of [[atomic structure]]. In a [[quantum mechanics|quantum mechanical]] description of atomic structure, this period corresponds to the filling of the [[atomic orbital|1s orbital]]. Period&nbsp;1 elements obey the [[octet rule#Exceptions|duet rule]] in that they need two [[electron]]s to complete their [[valence shell]].

Hydrogen and helium are [[Big Bang nucleosynthesis|the oldest]] and the most abundant elements in the [[universe]].

==Periodic trends==
All other periods in the periodic table contain at least eight elements, and it is often helpful to consider [[periodic trends]] across the period. However, period&nbsp;1 contains only two elements, so this concept does not apply here.{{fact|date=August 2019}}

In terms of vertical trends down groups, helium can be seen as a typical [[noble gas]] at the head of the IUPAC [[group&nbsp;18]], but as discussed below, hydrogen's chemistry is unique and it is not easily assigned to any group.<ref>{{cite journal|title=Where to Put Hydrogen in a Periodic Table?|journal=Foundations of Chemistry|year=2006|author=Michael Laing|doi=10.1007/s10698-006-9027-5|volume=9|issue=2|pages=127–137|s2cid=93781427 }}</ref>

==Position of period&nbsp;1 elements in the periodic table==
The first [[electron shell]], {{math|1=[[principal quantum number|''n'']] = 1}}, consists of only one orbital, and the maximum number of [[valence electron]]s that a period&nbsp;1 element can accommodate is two, both in the 1s orbital. The valence shell lacks "p" or any other kind of orbitals due to the [[Hydrogen-like_atom#Quantum_numbers|general {{math|''l'' < ''n''}} constraint]] on the [[quantum number]]s. Therefore, period&nbsp;1 has exactly two elements.
Although both hydrogen and helium are in the [[s-block]], neither of them behaves similarly to other s-block elements. Their behaviour is so different from the other s-block elements that there is considerable disagreement over where these two elements should be placed in the periodic table.

Simply following electron configurations, hydrogen (electronic configuration 1s<sup>1</sup>) and helium (1s<sup>2</sup>) should be placed in groups 1 and 2, above lithium (1s<sup>2</sup>2s<sup>1</sup>) and beryllium (1s<sup>2</sup>2s<sup>2</sup>).<ref name="Gray12">Gray, p. 12</ref> While such a placement is common for hydrogen, it is rarely used for helium outside of the context of illustrating the electron configurations. Usually, hydrogen is placed in group 1, and helium in group 18: this is the placement found on the IUPAC periodic table.<ref name="IUPAC-redbook">{{cite web |url=https://iupac.org/what-we-do/periodic-table-of-elements/ |title=Periodic Table of Elements |author=<!--Not stated--> |date=2021 |website=iupac.org |publisher=IUPAC |access-date=3 April 2021 |archive-date=10 April 2016 |archive-url=https://web.archive.org/web/20160410043726/https://iupac.org/what-we-do/periodic-table-of-elements/ |url-status=live }}</ref> Some variation can be found on both these matters.<ref name="KW" />

Like the group 1 metals, hydrogen has one electron in its outermost shell<ref name="Gray12">Gray, p. 12</ref> and typically loses its only electron in chemical reactions.<ref name="Vlasov" /> It has some metal-like chemical properties, being able to displace some metals from their [[salt (chemistry)|salts]].<ref name="Vlasov">{{cite book |last1=Vlasov |first1=L. |last2=Trifonov |first2=D. |translator-last1=Sobolev |translator-first1=D. |date=1970 |title=107 Stories About Chemistry |publisher=Mir Publishers |pages=23–27 |isbn=978-0-8285-5067-3}}</ref> But hydrogen forms a diatomic nonmetallic gas at standard conditions, unlike the alkali metals which are reactive solid metals. This and hydrogen's formation of [[hydride]]s, in which it gains an electron, brings it close to the properties of the [[halogen]]s which do the same<ref name=Vlasov/> (though it is rarer for hydrogen to form H<sup>−</sup> than H<sup>+</sup>).<ref name="raynercanham">{{cite book |last=Rayner-Canham |first=Geoffrey |date=2020 |title=The Periodic Table: Past, Present, Future |publisher=World Scientific |pages=71–84 |isbn=9789811218507}}</ref> Moreover, the lightest two halogens ([[fluorine]] and [[chlorine]]) are gaseous like hydrogen at standard conditions.<ref name="Vlasov" /> Some properties of hydrogen are not a good fit for either group: hydrogen is neither highly oxidising nor highly reducing and is not reactive with water.<ref name=raynercanham/> Hydrogen thus has properties corresponding to both those of the alkali metals and the halogens, but matches neither group perfectly, and is thus difficult to place by its chemistry.<ref name="Vlasov" /> Therefore, while the electronic placement of hydrogen in group 1 predominates, some rarer arrangements show either hydrogen in group 17,<ref>{{Clayden}}</ref> duplicate hydrogen in both groups 1 and 17,<ref>{{cite journal |last=Seaborg |first= G.|title=The chemical and radioactive properties of the heavy elements |journal= Chemical & Engineering News|year=1945 |volume=23 |issue=23 |pages=2190–93|doi= 10.1021/cen-v023n023.p2190}}</ref><ref name="Kaesz" /> or float it separately from all groups.<ref name="Kaesz">{{cite journal |last1=Kaesz |first1=Herb |last2=Atkins |first2=Peter |date=2009 |title=A Central Position for Hydrogen in the Periodic Table |journal=Chemistry International |volume=25 |issue=6 |page=14 |doi=10.1515/ci.2003.25.6.14 |doi-access=free }}</ref><ref name="GE">Greenwood & Earnshaw, throughout the book</ref><ref name="KW" /> The possibility of "floating" hydrogen has nonetheless been criticised by [[Eric Scerri]], who points out that removing it from all groups suggests that it is being excluded from the periodic law, when all elements should be subject to that law.<ref>{{cite journal |last1=Scerri |first1=Eric |date=2004 |title=The Placement of Hydrogen in the Periodic Table |url=https://old.iupac.org/publications/ci/2004/2603/ud2_scerri.html |journal=Chemistry International |volume=26 |issue=3 |pages=21–22 |doi=10.1515/ci.2004.26.3.21 |access-date=11 November 2022|doi-access=free |url-access=subscription }}</ref> A few authors have advocated more unusual placements for hydrogen, such as group 13 or group 14, on the grounds of trends in ionisation energy, electron affinity, and electronegativity.<ref name=raynercanham/>

Helium is an unreactive noble gas at standard conditions, and has a full outer shell: these properties are like the noble gases in group 18, but not at all like the reactive alkaline earth metals of group 2. Therefore, helium is nearly universally placed in group 18<ref name="IUPAC-redbook" /> which its properties best match.<ref name="KW" /> However, helium only has two outer electrons in its outer shell, whereas the other noble gases have eight; and it is an s-block element, whereas all other noble gases are p-block elements. Also, solid helium crystallises in a [[hexagonal close-packed]] structure, which matches beryllium and magnesium in group 2, but not the other noble gases in group 18.<ref name=Kurushkin /> In these ways helium better matches the alkaline earth metals.<ref name="Gray12" /><ref name="KW">{{cite book|last=Wothers|first=Peter|title=Chemical structure and reactivity : an integrated approach|year=2008|publisher=Oxford University Press|location=Oxford|isbn=978-0-19-928930-1|author2=Keeler, Wothers|page=259}}</ref> Therefore, tables with both hydrogen and helium floating outside all groups may rarely be encountered.<ref name="GE" /><ref name="KW" />

A few chemists, such as [[Henry Bent]], have advocated that the electronic placement in group 2 be adopted for helium.<ref name="PTSS" /><ref name=Kurushkin>{{cite journal |last1=Kurushkin |first1=Mikhail |date=2020 |title=Helium's placement in the Periodic Table from a crystal structure viewpoint |url=https://www.researchgate.net/publication/342152661 |journal=IUCrJ |volume=7 |issue=4 |pages=577–578 |doi=10.1107/S2052252520007769 |pmid=32695406 |pmc=7340260 |access-date=19 June 2020 |doi-access=free |bibcode=2020IUCrJ...7..577K |archive-date=19 October 2021 |archive-url=https://web.archive.org/web/20211019202250/https://www.researchgate.net/publication/342152661_Helium's_placement_in_the_Periodic_Table_from_a_crystal_structure_viewpoint |url-status=live }}</ref><ref>{{cite journal |last1=Grochala |first1=Wojciech |date=1 November 2017 |title=On the position of helium and neon in the Periodic Table of Elements |journal=Foundations of Chemistry |volume=20 |pages=191–207 |issue=2018 |doi=10.1007/s10698-017-9302-7 |doi-access=free }}</ref><ref>{{cite journal |last1=Bent Weberg |first1=Libby |date=18 January 2019 |title="The" periodic table |url=https://cen.acs.org/articles/97/i3/Reactions.html |journal=Chemical & Engineering News |volume=97 |issue=3 |access-date=27 March 2020 |archive-date=1 February 2020 |archive-url=https://web.archive.org/web/20200201200009/https://cen.acs.org/articles/97/i3/Reactions.html |url-status=live }}</ref><ref>{{cite journal |last1=Grandinetti |first1=Felice |date=23 April 2013 |title=Neon behind the signs |journal=Nature Chemistry |volume=5 |issue=2013 |page=438 |doi=10.1038/nchem.1631 |pmid=23609097 |bibcode=2013NatCh...5..438G |doi-access=free }}</ref> This assignment is also found in [[Charles Janet]]'s left-step table. Arguments for this often rest on the first-row anomaly trend (s >> p > d > f), which states that the first element of each group often behaves quite differently from the succeeding ones: the difference is largest in the s-block (H and He), is moderate for the p-block (B to Ne), and is less pronounced for the d- and f-blocks.<ref name=PTSS>[[Eric Scerri]]. 2020, ''The Periodic Table, Its Story and Its Significance'', 2nd edition, Oxford University Press, New York, {{ISBN|978-0190914363}}. pp. 392–401, 407–420.</ref> Thus helium as the first s<sup>2</sup> element before the alkaline earth metals stands out as anomalous in a way that helium as the first noble gas does not.<ref name="PTSS" /> The normalized ionization potentials and electron affinities show better trends with helium in group 2 than in group 18; helium is expected to be slightly more reactive than neon (which breaks the general trend of reactivity in the noble gases, where the heavier ones are more reactive); and predicted helium compounds often lack neon analogues even theoretically, but sometimes have beryllium analogues.<ref>{{cite journal |last1=Grochala |first1=Wojciech |date=1 November 2017 |title=On the position of helium and neon in the Periodic Table of Elements |journal=Foundations of Chemistry |volume=20 |pages=191–207 |issue=2018 |doi=10.1007/s10698-017-9302-7 |doi-access=free }}</ref><ref>{{cite journal |last1=Bent Weberg |first1=Libby |date=18 January 2019 |title="The" periodic table |url=https://cen.acs.org/articles/97/i3/Reactions.html |journal=Chemical & Engineering News |volume=97 |issue=3 |access-date=27 March 2020}}</ref><ref>{{cite journal |last1=Grandinetti |first1=Felice |date=23 April 2013 |title=Neon behind the signs |journal=Nature Chemistry |volume=5 |issue=2013 |pages=438 |doi=10.1038/nchem.1631 |pmid=23609097 |bibcode=2013NatCh...5..438G |doi-access=free }}</ref>

{{clear}}

==Elements==
{| class="wikitable"
|-
! colspan="3" | [[Chemical element]] !! [[Block (periodic table)|Block]] !! [[Electron configuration]]
|- style="background-color: {{element color|s-block}}"
| 1  || '''H'''  || [[Hydrogen]]   || [[s-block]]              || 1s<sup>1</sup>
|- style="background-color: {{element color|s-block}}"
| 2  || '''He''' || [[Helium]]      || [[s-block]]              || 1s<sup>2</sup>
|}

===Hydrogen===
{{Main|Hydrogen}}
[[File:Hydrogen discharge tube.jpg|thumb|150px|Hydrogen discharge tube]]
[[File:Deuterium discharge tube.jpg|thumb|150px|Deuterium discharge tube]]

Hydrogen (H) is the [[chemical element]] with [[atomic number]] 1. At [[standard temperature and pressure]], hydrogen is a colorless, odorless, [[Nonmetal (chemistry)|nonmetal]]lic, tasteless, highly [[combustion|flammable]] [[Diatomic molecule|diatomic]] [[gas]] with the [[molecular formula]] H<sub>2</sub>. With an [[atomic mass]] of 1.00794&nbsp;amu, hydrogen is the lightest element.<ref>{{cite web|access-date=2008-07-15|url=http://www.eia.doe.gov/kids/energyfacts/sources/IntermediateHydrogen.html|title=Hydrogen – Energy |publisher=Energy Information Administration}}</ref>

Hydrogen is the most [[abundance of the chemical elements|abundant]] of the chemical elements, constituting roughly 75% of the universe's elemental mass.<ref>{{cite web | last=Palmer | first=David | date=November 13, 1997 | url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/971113i.html | title=Hydrogen in the Universe | publisher=NASA | access-date=2008-02-05}}</ref> [[Star]]s in the [[main sequence]] are mainly composed of hydrogen in its [[plasma (physics)|plasma]] state. Elemental hydrogen is relatively rare on [[Earth]], and is industrially produced from [[hydrocarbon]]s such as methane, after which most elemental hydrogen is used "captively" (meaning locally at the production site), with the largest markets almost equally divided between [[fossil fuel]] upgrading, such as [[hydrocracking]], and [[ammonia]] production, mostly for the fertilizer market. Hydrogen may be produced from water using the process of [[electrolysis]], but this process is significantly more expensive commercially than [[hydrogen production]] from natural gas.<ref>{{cite web | author=Staff | year=2007 | url=http://www.fsec.ucf.edu/en/consumer/hydrogen/basics/production.htm | title=Hydrogen Basics&nbsp;— Production | publisher=Florida Solar Energy Center | access-date=2008-02-05}}</ref>

The most common naturally occurring [[isotope]] of hydrogen, known as [[hydrogen-1|protium]], has a single [[proton]] and no [[neutron]]s.<ref>{{cite news|title=Fusion Power Is Still Facing Formidable Difficulties|work=The New York Times|date=1971-03-11|author=Sullivan, Walter}}</ref> In [[ionic compound]]s, it can take on either a positive charge, becoming a [[Ion|cation]] composed of a bare proton, or a negative charge, becoming an [[Ion|anion]] known as a [[hydride]]. Hydrogen can form compounds with most elements and is present in [[water]] and most [[organic compound]]s.<ref>{{cite encyclopedia|encyclopedia=[[Encyclopædia Britannica]]|year=2008|title=hydrogen}}</ref> It plays a particularly important role in [[acid-base reaction theories|acid-base chemistry]], in which many reactions involve the exchange of protons between soluble molecules.<ref>{{cite journal|title=Electron-Driven Acid-Base Chemistry: Proton Transfer from Hydrogen Chloride to Ammonia|date=2008-02-15|volume=319|issue=5865|pages=936–939|doi=10.1126/science.1151614|author=Eustis, S. N.|journal=Science|pmid=18276886|last2=Radisic|first2=D.|last3=Bowen|first3=K. H.|last4=Bachorz|first4=R. A.|last5=Haranczyk|first5=M.|last6=Schenter|first6=G. K.|last7=Gutowski|first7=M.|bibcode = 2008Sci...319..936E |s2cid=29493053 }}</ref> As the only neutral atom for which the [[Schrödinger equation]] can be solved analytically, study of the energetics and [[atomic spectrum|spectrum]] of the hydrogen atom has played a key role in the development of [[quantum mechanics]].<ref>{{cite encyclopedia|encyclopedia=[[Encyclopædia Britannica]]|year=2008|title= Time-dependent Schrödinger equation}}</ref>

The interactions of hydrogen with various metals are very important in [[metallurgy]], as many metals can suffer [[hydrogen embrittlement]],<ref>{{cite journal | last=Rogers | first=H. C. | title=Hydrogen Embrittlement of Metals | journal=Science | year=1999 | volume=159 | issue=3819 | pages=1057–1064 | doi=10.1126/science.159.3819.1057 | pmid=17775040|bibcode = 1968Sci...159.1057R }}</ref> and in developing safe ways to store it for use as a fuel.<ref>{{cite news | last=Christensen | first=C. H. | author2=Nørskov, J. K. | author3=Johannessen, T. | date=July 9, 2005 | title=Making society independent of fossil fuels&nbsp;— Danish researchers reveal new technology | publisher=Technical University of Denmark | url=http://www.dtu.dk/English/About_DTU/News.aspx?guid=%7BE6FF7D39-1EDD-41A4-BC9A-20455C2CF1A7%7D | access-date=2008-03-28 | url-status=dead | archive-url=https://web.archive.org/web/20100107204859/http://www.dtu.dk/English/About_DTU/News.aspx?guid=%7BE6FF7D39-1EDD-41A4-BC9A-20455C2CF1A7%7D | archive-date=January 7, 2010 }}</ref> Hydrogen is highly soluble in many compounds composed of [[Rare earth element|rare earth metals]] and [[transition metal]]s<ref name="Takeshita">{{cite journal | last=Takeshita | first=T. |author2=Wallace, W.E. |author3=Craig, R.S.  | title=Hydrogen solubility in 1:5 compounds between yttrium or thorium and nickel or cobalt | journal=Inorganic Chemistry | year=1974 | volume=13 | issue=9 | pages=2282–2283 | doi = 10.1021/ic50139a050}}</ref> and can be dissolved in both [[crystalline]] and [[amorphous solid|amorphous]] metals.<ref name="Kirchheim1">{{cite journal | last=Kirchheim | first=R. |author2=Mutschele, T. |author3=Kieninger, W  | title=Hydrogen in amorphous and nanocrystalline metals | journal=Materials Science and Engineering | year=1988 | volume=99 | issue=1–2 | pages=457–462 | doi = 10.1016/0025-5416(88)90377-1}}</ref> Hydrogen solubility in metals is influenced by local distortions or impurities in the metal [[crystal lattice]].<ref name="Kirchheim2">{{cite journal | last=Kirchheim | first=R. | title=Hydrogen solubility and diffusivity in defective and amorphous metals | journal=Progress in Materials Science | year=1988 | volume=32 | issue=4 | pages=262–325 | doi = 10.1016/0079-6425(88)90010-2}}</ref>

===Helium===
{{Main|Helium}}
[[File:Helium discharge tube.jpg|thumb|upright|150px|Helium discharge tube]]

Helium (He) is a colorless, odorless, tasteless, non-toxic, [[Inert gas|inert]] [[monatomic]] chemical element that heads the [[noble gas]] series in the [[periodic table]] and whose [[atomic number]] is 2.<ref>{{cite web|access-date=2008-07-15|url=http://www.webelements.com/helium/|title=Helium: the essentials |publisher=WebElements}}</ref> Its [[boiling point|boiling]] and [[melting point|melting]] points are the lowest among the elements and it exists only as a [[gas]] except in extreme conditions.<ref>{{cite web|access-date=2008-07-15|url=http://www.webelements.com/helium/physics.html|title=Helium: physical properties |publisher=WebElements}}</ref>

Helium was discovered in 1868 by French astronomer [[Pierre Janssen]], who [[discovery of the chemical elements|first detected]] the substance as an unknown yellow [[spectroscopy|spectral line]] signature in light from a [[solar eclipse]].<ref>{{cite web|access-date=2008-07-15|url=http://encarta.msn.com/encyclopedia_762508746/pierre_janssen.html|title=Pierre Janssen|publisher=MSN Encarta|url-status=dead|archive-url=https://web.archive.org/web/20091029133529/http://encarta.msn.com/encyclopedia_762508746/Pierre_Janssen.html|archive-date=2009-10-29}}</ref> In 1903, large reserves of helium were found in the [[natural gas field]]s of the United States, which is by far the largest supplier of the gas.<ref>{{cite web|access-date=2008-07-15|url=http://www.blm.gov/wo/st/en/info/newsroom/2007/january/NR0701_2.html|title=Where Has All the Helium Gone?|publisher=Bureau of Land Management|date=2007-01-18|author=Theiss, Leslie|url-status=dead|archive-url=https://web.archive.org/web/20080725060842/http://www.blm.gov/wo/st/en/info/newsroom/2007/january/NR0701_2.html|archive-date=2008-07-25}}</ref> The substance is used in [[cryogenics]],<ref>{{cite book|title=Cryogenic Engineering: Fifty Years of Progress|author=Timmerhaus, Klaus D.|date=2006-10-06|publisher=Springer|isbn=0-387-33324-X}}</ref> in deep-sea breathing systems,<ref>{{cite journal|title=Helium voice unscrambling|author=Copel, M.|journal=Audio and Electroacoustics|volume=14|issue=3|date=September 1966|pages=122–126|doi=10.1109/TAU.1966.1161862}}</ref> to cool [[superconducting magnet]]s, in [[helium dating]],<ref>{{cite encyclopedia|encyclopedia=[[Encyclopædia Britannica]]|year=2008|title=helium dating}}</ref> for inflating [[balloon]]s,<ref>{{cite web|access-date=2008-07-15|url=http://www.howstuffworks.com/helium.htm|title=How Helium Balloons Work |publisher=How Stuff Works|author=Brain, Marshall|date=April 2000 }}</ref> for providing lift in [[airship]]s,<ref>{{cite web|access-date=2008-07-15|url=http://www.popsci.com/military-aviation-space/article/2008-07/return-blimp|title= The Return of the Blimp |work=Popular Science|date=2008-07-10|author=Jiwatram, Jaya}}</ref> and as a protective gas for industrial uses such as [[arc welding]] and growing [[silicon]] wafers.<ref>{{cite journal|title=When good GTAW arcs drift; drafty conditions are bad for welders and their GTAW arcs.|journal=Welding Design & Fabrication|date=2005-02-01}}</ref> Inhaling a small [[volume]] of the gas temporarily changes the timbre and quality of the human voice.<ref>{{cite web|access-date=2008-07-15|url=http://www.sciam.com/article.cfm?id=why-does-inhaling-helium|title=Why does inhaling helium make one's voice sound strange? |work=Scientific American|date=2006-09-04|author=Montgomery, Craig}}</ref> The behavior of liquid helium-4's two fluid phases, helium I and helium II, is important to researchers studying [[quantum mechanics]] and the phenomenon of [[superfluidity]] in particular,<ref>{{cite web|access-date=2008-07-15|url=https://www.sciencedaily.com/releases/2004/09/040903085531.htm|title=Probable Discovery Of A New, Supersolid, Phase Of Matter |publisher=Science Daily|date=2004-09-03}}</ref> and to those looking at the effects that temperatures near [[absolute zero]] have on [[matter]], such as with [[superconductivity]].<ref>{{cite news|title=Scientists See Peril In Wasting Helium; Scientists See Peril in Waste of Helium|work=The New York Times|date=1979-08-21|author= Browne, Malcolm W.}}</ref>

Helium is the second lightest element and is the second most [[chemical abundance|abundant]] in the observable universe.<ref>{{cite web|access-date=2008-07-15|url=http://www.webelements.com/helium/geology.html|title=Helium: geological information |publisher=WebElements}}</ref> Most helium was formed during the [[Big Bang]], but new helium is being created as a result of the [[nuclear fusion]] of hydrogen in [[star]]s.<ref>{{cite web|access-date=2008-07-15|url=https://www.newscientist.com/article/mg12517027.000-origin-of-the-chemical-elements.html|title=Origin of the chemical elements
|work=New Scientist|date=1990-02-03|author=Cox, Tony}}</ref> On [[Earth]], helium is relatively rare and is created by the natural [[radioactive decay|decay]] of some radioactive elements<ref>{{cite news|title=Helium supply deflated: production shortages mean some industries and partygoers must squeak by.|publisher=Houston Chronicle|date=2006-11-05}}</ref> because the [[alpha particle]]s that are emitted consist of helium [[atomic nucleus|nuclei]]. This radiogenic helium is trapped with [[natural gas]] in concentrations of up to seven percent by volume,<ref>{{cite web|access-date=2008-07-15|url=http://www.aapg.org/explorer/2008/02feb/helium.cfm|title=Helium a New Target in New Mexico |publisher=American Association of Petroleum Geologists|date=2008-02-02|author=Brown, David}}</ref> from which it is extracted commercially by a low-temperature separation process called [[fractional distillation]].<ref>{{cite news|title=Where Do We Get the Helium We Use?|publisher=The Science Teacher|date=2006-12-01|author=Voth, Greg}}</ref>

==References==
{{Reflist}}

==Further reading==
* {{cite book
 |author=Bloch, D. R.
 |year=2006
 |title=Organic Chemistry Demystified
 |url=https://books.google.com/books?id=yVPcSIn5xjAC
 |publisher=McGraw-Hill Professional
 |isbn=0-07-145920-0
 |ref=CITEREFBloch2006
}}

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[[Category:Periods (periodic table)]]