Alpha Particle

An alpha particle ($\alpha$) is a helium-4 nucleus: 2 protons and 2 neutrons, with a charge of +2.

Alpha particles are the most ionising type of radiation but the least penetrating. They are stopped by a few centimetres of air or a sheet of paper.

In alpha decay, the parent nucleus loses 2 protons and 2 neutrons, so its proton number decreases by 2 and its nucleon number decreases by 4.

${}^A_Z X \rightarrow {}^{A-4}_{Z-2} Y + {}^4_2 \alpha$