Editing Period 1 element (section)

==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>

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