During alpha decay, what happens to the atomic and mass numbers?

• A. Z decreases by 2; A decreases by 4
• B. Z increases by 2; A decreases by 4
• C. Z decreases by 0; A decreases by 4
• D. Z increases by 0; A decreases by 2

Answer: (A)

In alpha decay, the nucleus ejects an alpha particle, which is a helium-4 nucleus made of two protons and two neutrons. That means four nucleons leave the nucleus, so the mass number decreases by four. Since two protons are lost, the atomic number decreases by two. The resulting daughter nucleus thus has A reduced by 4 and Z reduced by 2. The emitted alpha particle also carries away charge, so the element shown on the periodic table moves to the element with two fewer protons.