Although there are nine known

isotopes Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

(₂He) ( standard atomic weight: ), only helium-3 () and

helium-4 Helium-4 () 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 consis ...

() are stable. All

radioisotopes A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...

are short-lived, the longest-lived being with a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...

of . The least stable is , with a half-life of (), although it is possible that may have an even shorter half-life. In the Earth's atmosphere, the ratio of to is . However, the isotopic abundance of helium varies greatly depending on its origin. In the Local Interstellar Cloud, the proportion of to is , which is times higher than that of atmospheric helium. Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in

geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...

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 and below separate into two immiscible phases due to differences in quantum statistics: atoms are

boson In particle physics, a boson ( ) is a subatomic particle whose spin quantum number has an integer value (0,1,2 ...). Bosons form one of the two fundamental classes of subatomic particle, the other being fermions, which have odd half-integer spi ...

s while atoms are fermions. Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.

List of isotopes

, - , rowspan=2, Intermediate in the

proton–proton chain The proton–proton chain, also commonly referred to as the chain, is one of two known sets of nuclear fusion reactions by which stars convert hydrogen to helium. It dominates in stars with masses less than or equal to that of the Sun, wherea ...

reaction , rowspan=2 style="text-align:right" , 2 , rowspan=2 style="text-align:right" , 0 , rowspan="2" , , rowspan=2 , ≪ , p (> ) , , rowspan=2 , 0+# , rowspan=2 , , rowspan=2 , , - , β⁺ (< ) , , - , Produced during

Big Bang nucleosynthesis In physical cosmology, Big Bang nucleosynthesis (abbreviated BBN, also known as primordial nucleosynthesis) is the production of nuclei other than those of the lightest isotope of hydrogen ( hydrogen-1, 1H, having a single proton as a nucleu ...

This and are the only stable nuclides with more protons than neutrons , style="text-align:right" , 2 , style="text-align:right" , 1 , , colspan=3 align=center, Stable , 1/2+ , , ref name="Isotopic Compositions of Elements 2013"> , - , , style="text-align:right" , 2 , style="text-align:right" , 2 , , colspan=3 align=center, Stable , 0+ , , ref name="Isotopic Compositions of Elements 2013" /> , - , , style="text-align:right" , 2 , style="text-align:right" , 3 , ,
[] , neutron emission, n , , 3/2− , , , - , rowspan=2, Has 2 halo nucleus, halo neutrons , rowspan=2 style="text-align:right" , 2 , rowspan=2 style="text-align:right" , 4 , rowspan=2, , rowspan=2, , β⁻ (%) , , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , β⁻d (%) , , - , , style="text-align:right" , 2 , style="text-align:right" , 5 , ,
[] , n , , (3/2)− , , , - , rowspan=3, Has 4 halo neutrons , rowspan=3 style="text-align:right" , 2 , rowspan=3 style="text-align:right" , 6 , rowspan=3, , rowspan=3, , β⁻ () , , rowspan=3, 0+ , rowspan=3, , rowspan=3, , - , β⁻n () , , - , β⁻t () , , - , , style="text-align:right" , 2 , style="text-align:right" , 7 , , , n , , 1/2(+) , , , - , , style="text-align:right" , 2 , style="text-align:right" , 8 , ,
[] , 2n , , 0+ , ,

Helium-2 (diproton)

Helium-2 or is an extremely unstable isotope of helium. Its nucleus, a diproton, consists of two Proton, protons with no neutrons. According to theoretical calculations, it would have been much more stable (although still undergoing positron emission, β⁺ decay to

deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...

) if the strong interaction had been 2% greater. Its instability is due to spin–spin interactions in the nuclear force, and the

Pauli exclusion principle In quantum mechanics, the Pauli exclusion principle states that two or more identical particles with half-integer spins (i.e. fermions) cannot occupy the same quantum state within a quantum system simultaneously. This principle was formulat ...

, which forces the two protons to have anti-aligned spins and gives the diproton a negative

binding energy In physics and chemistry, binding energy is the smallest amount of energy required to remove a particle from a system of particles or to disassemble a system of particles into individual parts. In the former meaning the term is predominantly use ...

. There may have been observations of . In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once—perhaps a 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 , 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 bundled together as a nucleus—which then decays into separate protons. Alternatively, the protons may be emitted separately but simultaneously—so-called "democratic decay". The experiment was not sensitive enough to establish which of these two processes was taking place. More evidence of was found in 2008 at the

Istituto Nazionale di Fisica Nucleare The Istituto Nazionale di Fisica Nucleare (INFN; "National Institute for Nuclear Physics") is the coordinating institution for nuclear, particle, theoretical and astroparticle physics in Italy. History INFN was founded on 8 August 1951, to furth ...

, in Italy. A beam of ions was directed at a target of beryllium foil. This collision converted some of the heavier neon nuclei in the beam into nuclei. These nuclei then collided with a foil of lead. The second collision had the effect of exciting the nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the nucleus decayed into an 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 the

Joint Institute for Nuclear Research The Joint Institute for Nuclear Research (JINR, russian: Объединённый институт ядерных исследований, ОИЯИ), in Dubna, Moscow Oblast (110 km north of Moscow), Russia, is an international research c ...

Dubna Dubna ( rus, Дубна́, p=dʊbˈna) is a town in Moscow Oblast, Russia. It has a status of ''naukograd'' (i.e. town of science), being home to the Joint Institute for Nuclear Research, an international nuclear physics research center and one o ...

, Russia, where beams of nuclei were directed at a cryogenic hydrogen target to produce . It was discovered that the nucleus can donate all four of its neutrons to the hydrogen. The two remaining protons could be simultaneously ejected from the target as a nucleus, which quickly decayed into two protons. A similar reaction has also been observed from nuclei colliding with hydrogen. is an intermediate in the first step of the

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 : + → + + . The hypothetical effect of the binding of the diproton on Big Bang and stellar nucleosynthesis has been investigated. Some models suggest that variations in the strong force allowing the existence of a bound diproton would enable the conversion of all primordial hydrogen to helium in the Big Bang, with catastrophic consequences on the development of stars and life. This proposition is used as an example of the anthropic principle. However, a 2009 study suggests that such a conclusion cannot be drawn, as the formed diprotons would still decay to deuterium, whose binding energy would also increase. In some scenarios, it is postulated that hydrogen (in the form of deuterium) could still survive in relatively large quantities, rebutting arguments that the strong force is tuned within a precise anthropic limit.

Helium-3

is stable and is the only stable isotope other than with more protons than neutrons. (There are many such unstable isotopes, the lightest being and .) There is only a trace amount () of 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 In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of ...

. In

stars A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth ma ...

, however, is more abundant, a product of

nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manife ...

. Extraplanetary material, such as lunar and asteroid

regolith Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestr ...

, has trace amounts of from

solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...

bombardment. For helium-3 to form a

superfluid Superfluidity is the characteristic property of a fluid with zero viscosity which therefore flows without any loss of kinetic energy. When stirred, a superfluid forms vortices that continue to rotate indefinitely. Superfluidity occurs in two ...

, it must be cooled to a temperature of , or almost a thousand times lower than helium-4 (). This difference is explained by quantum statistics, since helium-3 atoms are

fermions In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin , spin , etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks and ...

, while helium-4 atoms are bosons, which condense to a superfluid more easily.

Helium-4

The most common isotope, , is produced on Earth by

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an at ...

of heavier radioactive elements; the alpha particles that emerge are fully ionized nuclei. is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during

. Terrestrial helium consists almost exclusively () of this isotope. Helium-4's boiling point of is the second lowest of all known substances, second only to helium-3. When cooled further to , it transforms to a unique

state of zero

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

. It solidifies only at pressures above 25 atmospheres, where its melting point is .

Heavier helium isotopes

Although all heavier helium isotopes decay with a

of less than one second, researchers have used

particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for fundamental research in particle ...

collisions to create unusual atomic nuclei for elements such as helium,

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid ...

and

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

. The unusual nuclear structures of such isotopes may offer insight into the isolated properties of neutrons. The shortest-lived isotope is helium-10 with a

of . Helium-6 decays by emitting a beta particle and has a half-life of . The most widely studied heavy helium isotope is helium-8. This isotope, as well as helium-6, is thought to consist of a normal helium-4 nucleus surrounded by a neutron "halo" (containing two neutrons in and four neutrons in ). Halo nuclei have become an area of intense research. Isotopes up to helium-10, with two protons and eight neutrons, have been confirmed. , despite being a doubly magic isotope, has a very short half-life; it is not particle-bound and near-instantaneously drips out two neutrons.

References

External links

General Tables
— abstracts for helium and other exotic light nuclei {{Navbox element isotopes Helium

Helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...