Chemistry

In alpha decay, a parent nucleus ejects a helium-4 nucleus ($^4_2\text{He}$, or an alpha particle), which carries away 2 protons and 2 neutrons. Since the atomic number counts protons, losing 2 protons means the daughter nucleus has 2 fewer protons than the parent. This is why alpha decay always decreases the atomic number by exactly 2—it's a direct consequence of what leaves the nucleus.

In alpha decay, a parent nucleus ejects a helium-4 nucleus ($^4_2\text{He}$, or an alpha particle), which carries away 2 protons and 2 neutrons. Since the atomic number counts protons, losing 2 protons means the daughter nucleus has 2 fewer protons than the parent. This is why alpha decay always decreases the atomic number by exactly 2—it's a direct consequence of what leaves the nucleus.