Alpha Particle

SPIN: 0
SYMBOL: α
CHARGE: +2 e
MASS: 3727.3794118 ± 1.2 × 10⁻⁶ MeV/c²
CODATA SOURCE: CODATA 2022

Helium-4 nucleus of two protons and two neutrons — the doubly magic projectile of early nuclear physics.

Rutherford and Royds proved in 1909 that alpha particles are helium nuclei by collecting them and observing helium’s spectrum.
With two protons and two neutrons filling closed shells, it is exceptionally stable — which is why heavy nuclei shed alphas.
Alphas were the probe of the gold-foil experiment and of the first artificial nuclear transmutation.