two4He

Helium-4, commonly written as He-4 or 4He, is the most abundant isotope of the element helium. Its nucleus contains two protons and two neutrons (mass number A = 4, atomic number Z = 2) and is often referred to as an alpha particle. It is a stable, non-radioactive isotope with a binding energy of about 28.3 MeV, corresponding to roughly 7.07 MeV per nucleon. The nucleus has zero intrinsic spin.

Helium-4 was produced in the early universe during Big Bang nucleosynthesis and continues to form in stars through helium burning. In the cosmos, it is the dominant helium isotope, while on Earth nearly all natural helium is He-4. Terrestrial abundance in the atmosphere is very low, about 5 parts per million by volume, with most commercial helium extracted from natural gas deposits or produced industrially. Helium is light enough to escape Earth's atmosphere over geological timescales, contributing to its relative scarcity.

As a noble gas, He-4 is colorless, odorless, and chemically inert, existing as a monoatomic gas at standard conditions. It has a very low boiling point of 4.22 K and becomes a superfluid below 2.17 K, exhibiting unique quantum phenomena. The nucleus has zero spin, a property that influences its nuclear and spectroscopic behavior.

Applications of helium-4 include cryogenic cooling for superconducting magnets in MRI machines and in particle physics experiments, leak detection, and providing inert atmospheres. Its discovery is historically linked to its solar-origin name, with terrestrial isolation by Ramsay and Travers in 1895.