If an atom has too many protons, the nucleus can become unstable. What type of radioactive particle is ejected from the nucleus?

A beta particle is ejected

Alpha (α), beta (β-) and gamma (γ) decay. When the nucleus of an atom possesses either too many or too few neutrons compared to the number of protons it becomes unstable. These are called radioactive isotopes. Unstable nuclei split up in a process called radioactive decay and emit radioactive radiation. Source: BBC UK