Alpha decay

Alpha decay is when a helium nucleus is emitted from the isotope.

That means that 2 protons and 2 neutrons are emitted from the nucleus of the isotope.

Alpha decay equation

^A_Z X\,\,\rightarrow\,\,^{A-4}_{Z-2}Y\,\,+\,\,^4_2\alpha

Proton number change

When an atom emits alpha radiation, the proton number will decrease by 2.

That’s because it emits two protons, so now has 2 fewer protons.

Nucleon number change

When an atom emits alpha radiation, the nucleon number (mass number) decreases by 4.

This is because it loses two protons and two neutrons, so loses a total of 4 nucleons.