Editing Helium (section)

====Related stability of the helium-4 nucleus and electron shell====
The nucleus of the helium-4 atom is identical with an [[alpha particle]]. High-energy electron-scattering experiments show its charge to decrease exponentially from a maximum at a central point, exactly as does the charge density of helium's own [[electron cloud]]. This symmetry reflects similar underlying physics: the pair of neutrons and the pair of protons in helium's nucleus obey the same quantum mechanical rules as do helium's pair of electrons (although the nuclear particles are subject to a different nuclear binding potential), so that all these [[fermion]]s fully occupy 1s orbitals in pairs, none of them possessing orbital angular momentum, and each cancelling the other's intrinsic spin. This arrangement is thus energetically extremely stable for all these particles and has [[nuclear astrophysics|astrophysical]] implications.<ref name=Parker2022>{{cite journal |first1=M. C. |last1=Parker |first2=C. |last2=Jeynes |first3=W. N. |last3=Catford |title=Halo Properties in Helium Nuclei from the Perspective of Geometrical Thermodynamics |journal=Annalen der Physik |date=2022 |volume=534 |number=2100278 |doi=10.1002/andp.202100278 |doi-access=free|bibcode=2022AnP...53400278P }}</ref> Namely, adding another particle – proton, neutron, or alpha particle – would consume rather than release energy; all systems with [[mass number]] 5, as well as [[beryllium-8]] (comprising two alpha particles), are unbound.<ref name=8gap>{{cite journal |last1=Coc |first1=A. |last2=Vangioni |first2=E. |title=The triple-alpha reaction and the ''A''&nbsp;=&nbsp;8 gap in BBN and Population III stars |journal=Memorie della Società Astronomica Italiana |volume=85 |pages=124–129 |date=2014 |bibcode=2014MmSAI..85..124C |url=http://inspirehep.net/record/1338211/files/2014MmSAI..85..124C.pdf?version=1}}</ref>

For example, the stability and low energy of the electron cloud state in helium accounts for the element's chemical inertness, and also the lack of interaction of helium atoms with each other, producing the lowest melting and boiling points of all the elements. In a similar way, the particular energetic stability of the helium-4 nucleus, produced by similar effects, accounts for the ease of helium-4 production in atomic reactions that involve either heavy-particle emission or fusion. Some stable helium-3 (two protons and one neutron) is produced in fusion reactions from hydrogen, though its estimated abundance in the universe is about {{val|e=-5}} relative to helium-4.<ref name=Pitrou18/>

[[File:Binding energy curve - common isotopes.svg|thumb|right|Binding energy per nucleon of common isotopes. The binding energy per particle of helium-4 is significantly larger than all nearby nuclides.]]
The unusual stability of the helium-4 nucleus is also important [[Cosmology|cosmologically]]: it explains the fact that in the first few minutes after the [[Big Bang]], as the "soup" of free protons and neutrons which had initially been created in about 6:1 ratio cooled to the point that nuclear binding was possible, almost all first compound atomic nuclei to form were helium-4 nuclei. Owing to the relatively tight binding of helium-4 nuclei, its production consumed nearly all of the free neutrons in a few minutes, before they could beta-decay, and thus few neutrons were available to form heavier atoms such as lithium, beryllium, or boron. Helium-4 nuclear binding per nucleon is stronger than in any of these elements (see [[nucleogenesis]] and [[binding energy]]) and thus, once helium had been formed, no energetic drive was available to make elements 3, 4 and 5.<ref name=bbn99>{{cite journal |title=Cosmic lithium-beryllium-boron story |date=1999 |first1=E. |last1=Vangioni-Flam |first2=M. |last2=Cassé |doi=10.1023/A:1002197712862 |journal=Astrophysics and Space Science |volume=265 |pages=77–86 |arxiv=astro-ph/9902073|bibcode=1999Ap&SS.265...77V |s2cid=10627727 }}</ref> It is barely energetically favorable for helium to fuse into the next element with a lower energy per [[nucleon]], carbon. However, due to the short lifetime of the intermediate beryllium-8, this process requires three helium nuclei striking each other nearly simultaneously (see [[triple-alpha process]]).<ref name=8gap/> There was thus no time for significant carbon to be formed in the few minutes after the Big Bang, before the early expanding universe cooled to the temperature and pressure point where helium fusion to carbon was no longer possible. This left the early universe with a very similar ratio of hydrogen/helium as is observed today (3 parts hydrogen to 1 part helium-4 by mass), with nearly all the neutrons in the universe trapped in helium-4.

All heavier elements (including those necessary for rocky planets like the Earth, and for carbon-based or other life) have thus been created since the Big Bang in stars which were hot enough to fuse helium itself. All elements other than hydrogen and helium today account for only 2% of the mass of atomic matter in the universe. Helium-4, by contrast, comprises about 24% of the mass of the universe's ordinary matter—nearly all the ordinary matter that is not hydrogen.<ref name=Pitrou18>{{cite journal |title=Precision big bang nucleosynthesis with improved Helium-4 predictions |journal=Physics Reports |volume=754 |date=2018 |pages=1–66 |first1=C. |last1=Pitrou |first2=A. |last2=Coc |first3=J.-P. |last3=Uzan |first4=E. |last4=Vangioni |doi=10.1016/j.physrep.2018.04.005 |doi-access=free|arxiv=1801.08023 |bibcode=2018PhR...754....1P }}</ref><ref name=Hsyu20>{{cite journal |title=The PHLEK Survey: A New Determination of the Primordial Helium Abundance |first1=T. |last1=Hsyu |first2=R. J. |last2=Cooke |first3=J. X. |last3=Prochaska |first4=M. |last4=Bolte |date=2020 |journal=The Astrophysical Journal |volume=896 |number=77 |page=77 |doi=10.3847/1538-4357/ab91af |doi-access=free|arxiv=2005.12290 |bibcode=2020ApJ...896...77H }}</ref>