been known to be in the order of magnitude of 1 fm. In an experiment involving the use of exotic helium atoms where an atomic electron was replaced by a muon, the nucleus size has been estimated to be 1.67824(83) fm. Stability of the 4He nucleus and electron shell The nucleus of the helium-4 atom is identical to an alpha particle. High-energy electron-scattering experiments show its charge to decrease exponentially from a maximum at a central point, exactly as does the charge density of helium's own electron cloud. This symmetry reflects similar underlying physics: the pair of neutrons and the pair of protons in helium's nucleus obey the same quantum mechanical rules as do helium's pair of electrons (although the nuclear particles are subject to a different nuclear binding potential), so that all these fermions fully occupy 1s orbitals in pairs, none of them possessing orbital angular momentum, and each canceling the other's intrinsic spin. Adding another of any of these particles would require angular momentum, and would release substantially less energy (in fact, no nucleus with five nucleons is stable). This arrangement is thus energetically extremely stable for all these particles, and this stability accounts for many crucial facts regarding helium in nature. For example, the stability and low energy of the electron cloud of helium causes helium's chemical inertness (the most extreme of all the elements), and also the lack of interaction of helium atoms with each other (producing the lowest melting and boiling points of all the elements). In a similar way, the particular energetic stability of the helium-4 nucleus, produced by similar effects, accounts for the ease of helium-4 production in atomic reactions involving both heavy-particle emission and fusion. Some stable helium-3 is produced in fusion reactions from hydrogen, but it is a very small fraction, compared with the highly energetically favorable production of helium-4. The stability of helium-4 is the reason that hydrogen is converted to helium-4, and not deuterium (hydrogen-2) or helium-3 or other heavier elements during fusion reactions in the Sun. It is also partly responsible for the alpha particle being by far the most common type of baryonic particle to be ejected from an atomic nucleus; in other words, alpha decay is far more common than cluster decay. The unusual stability of the helium-4 nucleus is also important cosmologically. It explains the fact that, in the first few minutes after the Big