Standard atomic weight (Ar) 4.002602(2) | ||
 | ||
0.0002% is stable with 1 neutron 99.9998% is stable with 2 neutrons | ||
Although there are nine known isotopes of helium (2He) (standard atomic weight: 4.002602(2)), only helium-3 (3
He
) and helium-4 (4
He
) are stable. All radioisotopes are short-lived, the longest-lived being 6
He
with a half-life of 7002806700000000000♠806.7 milliseconds. The least stable is 5
He
, with a half-life of 6978760000000000000♠7.6×10−22 s, although it is possible that 2
He
has an even shorter half-life.
Contents
In the Earth's atmosphere, there is one 3
He
atom for every million 4
He
atoms. However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of 3
He
is around a hundred times higher. Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in geology to investigate the origin of rocks and the composition of the Earth's mantle. The different formation processes of the two stable isotopes of helium produce the differing isotope abundances.
Equal mixtures of liquid 3
He
and 4
He
below 6999800000000000000♠0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: 4
He
atoms are bosons while 3
He
atoms are fermions). Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.
Helium-2 (diproton)
Helium-2 or 2
He
, also known as a diproton, is an extremely unstable isotope of helium that consists of two protons with no neutrons. According to theoretical calculations it would have been much more stable (although still beta decaying to deuterium) if the strong force had been 2% greater. Its instability is due to spin–spin interactions in the nuclear force, and the Pauli exclusion principle, which forces the two protons to have anti-aligned spins and gives the diproton a negative binding energy.
There may have been observations of 2
He
. In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once—perhaps a 2
He
nucleus. The team led by Alfredo Galindo-Uribarri of the Oak Ridge National Laboratory announced that the discovery will help scientists understand the strong nuclear force and provide fresh insights into the creation of elements inside stars. Galindo-Uribarri and co-workers chose an isotope of neon with an energy structure that prevents it from emitting protons one at a time. This means that the two protons are ejected simultaneously. The team fired a beam of fluorine ions at a proton-rich target to produce 18
Ne
, which then decayed into oxygen and two protons. Any protons ejected from the target itself were identified by their characteristic energies. There are two ways in which the two-proton emission may proceed. The neon nucleus might eject a 'diproton'—a pair of protons bound together as a 2
He
nucleus—which then decays into separate protons. Alternatively, the protons may be emitted separately but at the same time—so-called 'democratic decay'. The experiment was not sensitive enough to establish which of these two processes was taking place.
More evidence of 2
He
was found in 2008 at the Istituto Nazionale di Fisica Nucleare, in Italy. A beam of 20
Ne
ions was directed at a target of beryllium foil. This collision converted some of the heavier neon nuclei in the beam into 18
Ne
nuclei. These nuclei then collided with a foil of lead. The second collision had the effect of exciting the 18
Ne
nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the 18
Ne
nucleus decayed into an 16
O
nucleus, plus two protons detected exiting from the same direction. The new experiment showed that the two protons were initially ejected together, correlated in a quasibound 1S configuration, before decaying into separate protons much less than a nanosecond later.
Further evidence comes from RIKEN in Japan and JINR in Dubna, Russia, where beams of 6
He
nuclei were directed at a cryogenic hydrogen target to produce 5
He
. It was discovered that the 6
He
nucleus can donate all four of its neutrons to the hydrogen. The two remaining protons could be simultaneously ejected from the target as a 2
He
nucleus, which quickly decayed into two protons. A similar reaction has also been observed from 8
He
nuclei colliding with hydrogen.
2
He
is an intermediate in the first step of the proton-proton chain reaction. The first step of the proton-proton chain reaction is a two-stage process; first, two protons fuse to form a diproton:
followed by the immediate beta-plus decay of the diproton to deuterium:
with the overall formula:
R. A. W. Bradford has considered the hypothetical effect of the binding of the diproton on Big Bang and stellar nucleosynthesis.
Helium-3
3
He
is stable, and is the only stable isotope other than 1
H
with more protons than neutrons. (There are many such unstable isotopes, the lightest being 7
Be
and 8
B
.) There is only a trace amount (0.000137%) of 3
He
on Earth, primarily present since the formation of the Earth, although some falls to Earth trapped in cosmic dust. Trace amounts are also produced by the beta decay of tritium. In stars, however, 3
He
is more abundant, a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, has trace amounts of 3
He
from bombardment with solar wind.
For helium-3 to form a superfluid, it must be cooled to a temperature of 0.0025 K, or almost a thousand times lower than helium-4 (2.17 K). This difference is explained by quantum statistics, since helium-3 atoms are fermions while helium-4 atoms are bosons, which condense to a superfluid more easily.
Helium-4
The most common isotope, 4
He
, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized 4
He
nuclei. 4
He
is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis.
Terrestrial helium consists almost exclusively (99.99986%) of this isotope. Its boiling point of 4.2 K is the lowest of any known substance. When cooled further to 2.17 K, it transforms to a unique superfluid state of zero viscosity. It solidifies only at pressures above 25 atmospheres, where its melting point is 0.95 K.
Heavier helium isotopes
Although all heavier helium isotopes decay with a half-life of less than one second, researchers have created new isotopes through particle accelerator collisions to create unusual atomic nuclei for elements such as helium, lithium and nitrogen. The unusual nuclear structures of such isotopes may offer insight into the isolated properties of neutrons.
The shortest-lived isotope is helium-5 with a half-life of 7.6×10−22 seconds. Helium-6 decays by emitting a beta particle and has a half-life of 0.8 seconds. Helium-7 also emits a beta particle as well as a gamma ray. The most widely studied heavy helium isotope is helium-8. This isotope, as well as helium-6, are thought to consist of a normal helium-4 nucleus surrounded by a neutron "halo" (containing two neutrons in 6
He
and four neutrons in 8
He
). Halo nuclei have become an area of intense research. Isotopes up to helium-10, with two protons and eight neutrons, have been confirmed. Helium-7 and helium-8 are hyperfragments that are created in certain nuclear reactions. 10
He
, despite being a doubly magic isotope, has a very short half-life as it is not particle-bound and near-instantaneously drips out two neutrons.
Notes
Decay chains
Although some helium isotopes, such as 6He and 8He, decay mostly to isotopes of lithium, the major tendency among known isotopes seems to be decay into lighter helium isotopes. Fission, seen only in even-numbered isotopes, is also unusually common.
Decay chains for isotopes with multiple decay modes listed in order of probability.