Alpha particle mass

Rutherford proved in 1908 that alpha rays are helium nuclei by collecting them in an evacuated tube and firing a spark through it to see the helium spectrum; three years later the same particles, bouncing backwards off gold foil, revealed the nucleus itself. Two protons and two neutrons make a doubly magic, exceptionally tight package: 28.296 MeV of binding, 7.07 MeV per nucleon, far above its neighbours.

That stability is why alpha decay exists at all — heavy nuclei can release energy by emitting a ready-made helium nucleus, tunnelling out through the Coulomb barrier — and why stellar nucleosynthesis proceeds in alpha-sized steps to ¹²C, ¹⁶O, ²⁰Ne. Note the mass here is the bare nucleus; a neutral ⁴He atom is 4.002 602 u, heavier by two electrons. Alphas are also intensely ionising but stopped by paper or skin, which makes them harmless outside the body and dangerous inside it.