4He

Helium-4 (4He) is the most abundant isotope of the element helium. Its nucleus contains two protons and two neutrons, yielding a mass number of 4 and an atomic number of 2. It is stable, non-radioactive, and, because its two electrons fill the 1s orbital, it behaves as a chemically inert noble gas under ordinary conditions.

Nuclear properties: 4He has spin 0 and is often regarded as a doubly magic nucleus (Z = 2, N = 2). Its nuclear binding energy is about 28.3 MeV, making the alpha particle highly tightly bound. Most production of 4He on Earth arises from alpha decay of heavier elements and from natural gas sources; in the universe, it is also produced in stellar nucleosynthesis and during Big Bang nucleosynthesis.

Physical properties: At standard pressure, 4He is a gas with the lowest boiling point of all elements (4.216 K). It can be liquefied and solidified only at cryogenic temperatures and, below about 2.17 K at saturated vapor pressure, becomes a superfluid (He II). Its chemical inertness and low density make it useful for leak detection and for shielding in certain experiments.

Occurrence and uses: On Earth, nearly all natural helium is 4He; 3He is comparatively rare. Industrially, helium-4 is extracted from natural gas reservoirs or separated from air by cryogenic methods. It is widely used in cryogenics, especially to cool superconducting magnets in MRI machines and in research facilities, and in low-temperature experimental apparatus. It is also employed as an inert shielding gas in welding and as a carrier gas in certain analytical techniques.