Description: If we are talking about radioactivity, it is because we are talking about the nucleus, that is to say protons and neutrons which are held together by a strong interaction which ensures the cohesion of the nucleus.
If a nucleus has too much matter, or energy, it will disintegrate, that is, it will seek to get rid of it. We say he's unstable. In order to seek to become stable, he will relieve himself of what encumbers him.
There are three kinds of radioactivity, alpha beta and gamma. We can even count 4 since there is beta plus and beta minus.
We'll also talk about the valley of stability.

Definition

Radioactive decay: process by which an unstable atomic nucleus loses matter or energy by radiation.
A radioactive material containing unstable nuclei.
Three of the most common types of decay are alpha decay (α-decay), beta decay (β-decay), and gamma decay (γ-decay).