Alpha particle emission is a type of radioactive decay where an unstable atomic nucleus releases an alpha particle. An alpha particle consists of two protons and two neutrons, making it identical to a helium nucleus. This process reduces the original nucleus's atomic number by two and its mass number by four, transforming it into a different element. Alpha particles have low penetration power and can be stopped by a sheet of paper or the outer layer of human skin. However, if ingested or inhaled, they can cause significant damage to biological tissues due to their high mass and charge.