Period 1 element

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 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 18, but as discussed below, hydrogen's chemistry is unique and it is not easily assigned to any group.{{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 }}

The first electron shell, {{math|1=n = 1}}, consists of only one orbital, and the maximum number of valence electrons that a period 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 on the quantum numbers. Therefore, period 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 1s¹) and helium (1s²) should be placed in groups 1 and 2, above lithium (1s²2s¹) and beryllium (1s²2s²).Gray, p. 12 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.{{cite web |url=https://iupac.org/what-we-do/periodic-table-of-elements/ |title=Periodic Table of Elements |author= |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 }} Some variation can be found on both these matters.

Like the group 1 metals, hydrogen has one electron in its outermost shellGray, p. 12 and typically loses its only electron in chemical reactions. It has some metal-like chemical properties, being able to displace some metals from their salts.{{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}} 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 hydrides, in which it gains an electron, brings it close to the properties of the halogens which do the same (though it is rarer for hydrogen to form H⁻ than H⁺).{{cite book |last=Rayner-Canham |first=Geoffrey |date=2020 |title=The Periodic Table: Past, Present, Future |publisher=World Scientific |pages=71–84 |isbn=9789811218507}} Moreover, the lightest two halogens (fluorine and chlorine) are gaseous like hydrogen at standard conditions. 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. 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. Therefore, while the electronic placement of hydrogen in group 1 predominates, some rarer arrangements show either hydrogen in group 17,{{Clayden}} duplicate hydrogen in both groups 1 and 17,{{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}} or float it separately from all groups.{{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 }}Greenwood & Earnshaw, throughout the book 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.{{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 }} 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.

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 which its properties best match. 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. In these ways helium better matches the alkaline earth metals.{{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}} Therefore, tables with both hydrogen and helium floating outside all groups may rarely be encountered.

A few chemists, such as Henry Bent, have advocated that the electronic placement in group 2 be adopted for helium.{{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 }}{{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 }}{{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 }}{{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 }} 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.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. Thus helium as the first s² element before the alkaline earth metals stands out as anomalous in a way that helium as the first noble gas does not. 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.{{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 }}{{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}}{{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 }}

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{{Main|Hydrogen}}

File:Hydrogen discharge tube.jpg

File:Deuterium discharge tube.jpg

Hydrogen (H) is the chemical element with atomic number 1. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H₂. With an atomic mass of 1.00794 amu, hydrogen is the lightest element.{{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}}

Hydrogen is the most abundant of the chemical elements, constituting roughly 75% of the universe's elemental mass.{{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}} Stars in the main sequence are mainly composed of hydrogen in its plasma state. Elemental hydrogen is relatively rare on Earth, and is industrially produced from hydrocarbons 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.{{cite web | author=Staff | year=2007 | url=http://www.fsec.ucf.edu/en/consumer/hydrogen/basics/production.htm | title=Hydrogen Basics — Production | publisher=Florida Solar Energy Center | access-date=2008-02-05}}

The most common naturally occurring isotope of hydrogen, known as protium, has a single proton and no neutrons.{{cite news|title=Fusion Power Is Still Facing Formidable Difficulties|work=The New York Times|date=1971-03-11|author=Sullivan, Walter}} In ionic compounds, it can take on either a positive charge, becoming a cation composed of a bare proton, or a negative charge, becoming an anion known as a hydride. Hydrogen can form compounds with most elements and is present in water and most organic compounds.{{cite encyclopedia|encyclopedia=Encyclopædia Britannica|year=2008|title=hydrogen}} It plays a particularly important role in acid-base chemistry, in which many reactions involve the exchange of protons between soluble molecules.{{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 }} As the only neutral atom for which the Schrödinger equation can be solved analytically, study of the energetics and spectrum of the hydrogen atom has played a key role in the development of quantum mechanics.{{cite encyclopedia|encyclopedia=Encyclopædia Britannica|year=2008|title= Time-dependent Schrödinger equation}}

The interactions of hydrogen with various metals are very important in metallurgy, as many metals can suffer hydrogen embrittlement,{{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 }} and in developing safe ways to store it for use as a fuel.{{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 — 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 }} Hydrogen is highly soluble in many compounds composed of rare earth metals and transition metals{{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}} and can be dissolved in both crystalline and amorphous metals.{{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}} Hydrogen solubility in metals is influenced by local distortions or impurities in the metal crystal lattice.{{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}}

{{Main|Helium}}

File:Helium discharge tube.jpg

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

Helium was discovered in 1868 by French astronomer Pierre Janssen, who first detected the substance as an unknown yellow spectral line signature in light from a solar eclipse.{{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}} In 1903, large reserves of helium were found in the natural gas fields of the United States, which is by far the largest supplier of the gas.{{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}} The substance is used in cryogenics,{{cite book|title=Cryogenic Engineering: Fifty Years of Progress|author=Timmerhaus, Klaus D.|date=2006-10-06|publisher=Springer|isbn=0-387-33324-X}} in deep-sea breathing systems,{{cite journal|title=Helium voice unscrambling|author=Copel, M.|journal= IEEE Transactions on Audio and Electroacoustics|volume=14|issue=3|date=September 1966|pages=122–126|doi=10.1109/TAU.1966.1161862}} to cool superconducting magnets, in helium dating,{{cite encyclopedia|encyclopedia=Encyclopædia Britannica|year=2008|title=helium dating}} for inflating balloons,{{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 }} for providing lift in airships,{{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}} and as a protective gas for industrial uses such as arc welding and growing silicon wafers.{{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}} Inhaling a small volume of the gas temporarily changes the timbre and quality of the human voice.{{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}} 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,{{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}} and to those looking at the effects that temperatures near absolute zero have on matter, such as with superconductivity.{{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.}}

Helium is the second lightest element and is the second most abundant in the observable universe.{{cite web|access-date=2008-07-15|url=http://www.webelements.com/helium/geology.html|title=Helium: geological information |publisher=WebElements}} Most helium was formed during the Big Bang, but new helium is being created as a result of the nuclear fusion of hydrogen in stars.{{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}} On Earth, helium is relatively rare and is created by the natural decay of some radioactive elements{{cite news|title=Helium supply deflated: production shortages mean some industries and partygoers must squeak by.|publisher=Houston Chronicle|date=2006-11-05}} because the alpha particles that are emitted consist of helium nuclei. This radiogenic helium is trapped with natural gas in concentrations of up to seven percent by volume,{{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}} from which it is extracted commercially by a low-temperature separation process called fractional distillation.{{cite news|title=Where Do We Get the Helium We Use?|publisher=The Science Teacher|date=2006-12-01|author=Voth, Greg}}

{{Reflist}}

• {{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)