Radioactive Decay

An unstable nucleus throws out a particle or energy and becomes another nucleus: radioactive decay.

In alpha decay it throws out an α particle, 2 protons and 2 neutrons: a helium-4 nucleus. $Z$ drops by 2 and $A$ by 4.

$${}^{238}_{92}\mathrm{U} \to {}^{234}_{90}\mathrm{Th} + {}^{4}_{2}\mathrm{He}$$

Alpha decay: the nucleus throws out two protons and two neutrons

In beta decay a neutron becomes a proton, and an electron, the β particle, leaves the nucleus. $Z$ rises by 1; $A$ stays the same.

$${}^{14}_{6}\mathrm{C} \to {}^{14}_{7}\mathrm{N} + {}^{0}_{-1}\mathrm{e}$$

Beta decay: a neutron becomes a proton, and an electron leaves

Gamma rays are energy only, so $A$ and $Z$ do not change. A decay equation balances: the mass numbers on the left add up to those on the right, and so do the atomic numbers. For uranium: 238 = 234 + 4 and 92 = 90 + 2.