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Helium-4 (4
He
) is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consists of two protons and two neutrons.
 | |
| General | |
|---|---|
| Symbol | 4He |
| Names | helium-4, 4He, He-4 |
| Protons (Z) | 2 |
| Neutrons (N) | 2 |
| Nuclide data | |
| Natural abundance | 99.999863% |
| Half-life (t1/2) | stable |
| Isotope mass | 4.002603254 Da |
| Spin | 0 |
| Binding energy | 28295.7 keV |
| Isotopes of helium Complete table of nuclides | |
Helium-4 makes up about one quarter of the ordinary matter in the universe by mass, with almost all of the rest being hydrogen. While nuclear fusion in stars also produces helium-4, most of the helium-4 in the Sun and in the universe is thought to have been produced during the Big Bang, known as "primordial helium". However, primordial helium-4 is largely absent from the Earth, having escaped during the high-temperature phase of Earth's formation. On Earth, most naturally occurring helium-4 is produced by the alpha decay of heavy elements in the Earth's crust, after the planet cooled and solidified.
When liquid helium-4 is cooled to below 2.17 K (−270.98 °C), it becomes a superfluid, with properties very different from those of an ordinary liquid. For example, if superfluid helium-4 is placed in an open vessel, a thin Rollin film will climb the sides of the vessel, causing the liquid to escape. The total spin of the helium-4 nucleus is an integer (zero), making it a boson. The superfluid behavior is a manifestation of Bose–Einstein condensation, which occurs only in collections of bosons.
It is theorized that at 0.2 K and 50 atm, solid helium-4 may be a superglass (an amorphous solid exhibiting superfluidity).
The helium-4 atom
The helium atom is the second simplest atom (hydrogen is the simplest), but the extra electron introduces a third "body", so its wave equation becomes a "three-body problem", which has no analytic solution. However, numerical approximations of the equations of quantum mechanics have given a good estimate of the key atomic properties of helium-4, such as its size and ionization energy.
The size of the 4He nucleus has long been known to be in the order of magnitude of 1 fm. In an experiment involving the use of exotic helium atoms where an atomic electron was replaced by a muon, the nucleus size has been estimated to be 1.67824(83) fm.
Stability of the 4He nucleus and electron shell
The nucleus of the helium-4 atom has a type of stability called doubly magic. 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 fermions fully occupy 1s orbitals in pairs, none of them possessing orbital angular momentum, and each canceling the other's intrinsic spin. Adding another of any of these particles would require angular momentum, and would release substantially less energy (in fact, no nucleus with five nucleons is stable). This arrangement is thus energetically extremely stable for all these particles, and this stability accounts for many crucial facts regarding helium in nature.
For example, the stability and low energy of the electron cloud of helium causes helium's chemical inertness (the most extreme of all the elements), 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 involving both heavy-particle emission and fusion. Some stable helium-3 is produced in fusion reactions from hydrogen, but it is a very small fraction, compared with the highly energetically favorable production of helium-4. The stability of helium-4 is the reason that hydrogen is converted to helium-4, and not deuterium (hydrogen-2) or helium-3 or other heavier elements during fusion reactions in the Sun. It is also partly responsible for the alpha particle being by far the most common type of baryonic particle to be ejected from an atomic nucleus; in other words, alpha decay is far more common than cluster decay.
The unusual stability of the helium-4 nucleus is also important 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 a 6:1 ratio cooled to the point where nuclear binding was possible, almost all atomic nuclei to form were helium-4 nuclei. The binding of the nucleons in helium-4 is so tight that its production consumed nearly all the free neutrons in a few minutes, before they could beta decay, and left very few to form heavier atoms (especially lithium, beryllium, and boron). The energy of helium-4 nuclear binding per nucleon is stronger than in any of those elements (see nucleogenesis and binding energy), and thus no energetic "drive" was available to make elements 3, 4, and 5 once helium had been formed. It is barely energetically favorable for helium to fuse into the next element with a higher energy per nucleon (carbon). However, due to the rarity of intermediate elements, and extreme instability of beryllium-8 (the product when two 4He nuclei fuse), this process needs three helium nuclei striking each other nearly simultaneously (see triple-alpha process). 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 where helium fusion to carbon was no longer possible. This left the early universe with a very similar hydrogen–helium ratio 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—thus had to be produced, since the Big Bang, in stars which were hot enough to fuse elements heavier than hydrogen. 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, makes up about 23% of the universe's ordinary matter—nearly all the ordinary matter that is not hydrogen (1H).
See also
- Big Bang nucleosynthesis
References
- Giulio Biroli; Claudio Chamon; Francesco Zamponi (2008). "Theory of the superglass phase". Physical Review B. 78 (22): 19. arXiv:0807.2458. Bibcode:2008PhRvB..78v4306B. doi:10.1103/PhysRevB.78.224306. S2CID 3222218.
- "Press release: Supersolid or superglass? Cornell researchers study a strange state of matter in helium - Cornell Chronicle".
- Yu, Xiaoquan; Mueller, Markus (2012). "Mean field theory of superglasses". Physical Review B. 85 (10): 104205. arXiv:1111.5956. Bibcode:2012PhRvB..85j4205Y. doi:10.1103/PhysRevB.85.104205. S2CID 119261743.
- Julian J. Krauth; Schuhmann, Karsten; Ahmed, Marwan Abdou; et al. (2021). "Measuring the α-particle charge radius with muonic helium-4 ions". Nature. 589 (7843): 527–531. Bibcode:2021Natur.589..527K. doi:10.1038/s41586-021-03183-1. PMC 7914124. PMID 33505036.
External links
- Superfluid Helium-4 Interactive Properties
- Tur, Clarisse (2009), "DEPENDENCE OF s-PROCESS NUCLEOSYNTHESIS IN MASSIVE STARS ON TRIPLE-ALPHA AND 12
C
(α, γ)16
O
REACTION RATE UNCERTAINTIES", The Astrophysical Journal, 702 (2): 1068–1077, arXiv:0809.0291, Bibcode:2009ApJ...702.1068T, doi:10.1088/0004-637x/702/2/1068, S2CID 118483505
This article needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Helium 4 news newspapers books scholar JSTOR December 2023 Learn how and when to remove this message Helium 4 4 He is a stable isotope of the element helium It is by far the more abundant of the two naturally occurring isotopes of helium making up about 99 99986 of the helium on Earth Its nucleus is identical to an alpha particle and consists of two protons and two neutrons Helium 4 4HeGeneralSymbol4HeNameshelium 4 4He He 4Protons Z 2Neutrons N 2Nuclide dataNatural abundance99 999863 Half life t1 2 stableIsotope mass4 002603254 DaSpin0Binding energy28295 7 keVIsotopes of helium Complete table of nuclidesThe helium atom Depicted are the nucleus pink and the electron cloud distribution black The nucleus upper right in helium 4 is in reality spherically symmetric and closely resembles the electron cloud although for more complicated nuclei this is not always the case Helium 4 makes up about one quarter of the ordinary matter in the universe by mass with almost all of the rest being hydrogen While nuclear fusion in stars also produces helium 4 most of the helium 4 in the Sun and in the universe is thought to have been produced during the Big Bang known as primordial helium However primordial helium 4 is largely absent from the Earth having escaped during the high temperature phase of Earth s formation On Earth most naturally occurring helium 4 is produced by the alpha decay of heavy elements in the Earth s crust after the planet cooled and solidified When liquid helium 4 is cooled to below 2 17 K 270 98 C it becomes a superfluid with properties very different from those of an ordinary liquid For example if superfluid helium 4 is placed in an open vessel a thin Rollin film will climb the sides of the vessel causing the liquid to escape The total spin of the helium 4 nucleus is an integer zero making it a boson The superfluid behavior is a manifestation of Bose Einstein condensation which occurs only in collections of bosons It is theorized that at 0 2 K and 50 atm solid helium 4 may be a superglass an amorphous solid exhibiting superfluidity The helium 4 atomThe helium atom is the second simplest atom hydrogen is the simplest but the extra electron introduces a third body so its wave equation becomes a three body problem which has no analytic solution However numerical approximations of the equations of quantum mechanics have given a good estimate of the key atomic properties of helium 4 such as its size and ionization energy The size of the 4He nucleus has long been known to be in the order of magnitude of 1 fm In an experiment involving the use of exotic helium atoms where an atomic electron was replaced by a muon the nucleus size has been estimated to be 1 67824 83 fm Stability of the 4He nucleus and electron shellThe nucleus of the helium 4 atom has a type of stability called doubly magic 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 fermions fully occupy 1s orbitals in pairs none of them possessing orbital angular momentum and each canceling the other s intrinsic spin Adding another of any of these particles would require angular momentum and would release substantially less energy in fact no nucleus with five nucleons is stable This arrangement is thus energetically extremely stable for all these particles and this stability accounts for many crucial facts regarding helium in nature For example the stability and low energy of the electron cloud of helium causes helium s chemical inertness the most extreme of all the elements 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 involving both heavy particle emission and fusion Some stable helium 3 is produced in fusion reactions from hydrogen but it is a very small fraction compared with the highly energetically favorable production of helium 4 The stability of helium 4 is the reason that hydrogen is converted to helium 4 and not deuterium hydrogen 2 or helium 3 or other heavier elements during fusion reactions in the Sun It is also partly responsible for the alpha particle being by far the most common type of baryonic particle to be ejected from an atomic nucleus in other words alpha decay is far more common than cluster decay 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 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 a 6 1 ratio cooled to the point where nuclear binding was possible almost all atomic nuclei to form were helium 4 nuclei The binding of the nucleons in helium 4 is so tight that its production consumed nearly all the free neutrons in a few minutes before they could beta decay and left very few to form heavier atoms especially lithium beryllium and boron The energy of helium 4 nuclear binding per nucleon is stronger than in any of those elements see nucleogenesis and binding energy and thus no energetic drive was available to make elements 3 4 and 5 once helium had been formed It is barely energetically favorable for helium to fuse into the next element with a higher energy per nucleon carbon However due to the rarity of intermediate elements and extreme instability of beryllium 8 the product when two 4He nuclei fuse this process needs three helium nuclei striking each other nearly simultaneously see triple alpha process 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 where helium fusion to carbon was no longer possible This left the early universe with a very similar hydrogen helium ratio 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 thus had to be produced since the Big Bang in stars which were hot enough to fuse elements heavier than hydrogen 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 makes up about 23 of the universe s ordinary matter nearly all the ordinary matter that is not hydrogen 1H See alsoBig Bang nucleosynthesisReferencesGiulio Biroli Claudio Chamon Francesco Zamponi 2008 Theory of the superglass phase Physical Review B 78 22 19 arXiv 0807 2458 Bibcode 2008PhRvB 78v4306B doi 10 1103 PhysRevB 78 224306 S2CID 3222218 Press release Supersolid or superglass Cornell researchers study a strange state of matter in helium Cornell Chronicle Yu Xiaoquan Mueller Markus 2012 Mean field theory of superglasses Physical Review B 85 10 104205 arXiv 1111 5956 Bibcode 2012PhRvB 85j4205Y doi 10 1103 PhysRevB 85 104205 S2CID 119261743 Julian J Krauth Schuhmann Karsten Ahmed Marwan Abdou et al 2021 Measuring the a particle charge radius with muonic helium 4 ions Nature 589 7843 527 531 Bibcode 2021Natur 589 527K doi 10 1038 s41586 021 03183 1 PMC 7914124 PMID 33505036 External linksSuperfluid Helium 4 Interactive Properties Tur Clarisse 2009 DEPENDENCE OF s PROCESS NUCLEOSYNTHESIS IN MASSIVE STARS ON TRIPLE ALPHA AND 12 C a g 16 O REACTION RATE UNCERTAINTIES The Astrophysical Journal 702 2 1068 1077 arXiv 0809 0291 Bibcode 2009ApJ 702 1068T doi 10 1088 0004 637x 702 2 1068 S2CID 118483505 Lighter helium 3 Helium 4 is an isotope of helium Heavier helium 5Decay product of lithium 5 p helium 5 n beryllium 6 2p beryllium 8 a Decay chain of helium 4 Decays to Stable
