He4

Helium-4, often written as He-4, is the most common isotope of helium. Its nucleus contains two protons and two neutrons, giving it a mass number A of 4 and an atomic number Z of 2. The nucleus is commonly called an alpha particle in nuclear physics.

In terms of basic properties, a helium-4 atom has a mass of about 4.0026 atomic mass units and a +2e charge. Its binding energy is about 28.3 MeV, corresponding to roughly 7.07 MeV per nucleon, making it a very tightly bound, stable nucleus. Helium-4 is stable and does not undergo radioactive decay; the neutral atom is a colorless, inert noble gas at room temperature and pressure.

Helium-4 is produced abundantly in the universe and on Earth. In the Big Bang nucleosynthesis, about 25% of the mass of ordinary matter formed as helium-4, making it the most prevalent helium isotope in the cosmos. Terrestrially, helium is almost entirely helium-4, with trace amounts of helium-3 from cosmic rays and other sources.

In stellar environments, helium-4 is produced by hydrogen fusion in the cores of stars. It also participates in the triple-alpha process, wherein three helium-4 nuclei combine in high-temperature environments to form carbon-12, releasing energy in the process. He-4 is also emitted as an alpha particle in the alpha decay of heavy elements.

Applications of helium-4 include its use as an inert cover gas, in cryogenics (notably for cooling superconducting magnets in MRI machines), and in various scientific and industrial contexts. Its stable, highly resistant nature to decay makes it a common component of natural helium and a useful particle in nuclear physics experiments.