Radioactive Decay

Radioactive decay is a change inside an unstable nucleus. The nucleus gives out radiation to become more stable. This happens on its own (spontaneous) and we cannot predict exactly when for any single atom (random).

Why some nuclei decay

Atoms of the same element can have different numbers of neutrons. These are called isotopes. Some isotopes have too many neutrons or a nucleus that is too large. They decay to reduce this and increase stability.

Types of radiation from the nucleus

• Alpha (α): a particle made of 2 protons and 2 neutrons (same as a helium nucleus). After α-decay, mass number decreases by 4 and atomic number decreases by 2.
• Beta (β⁻): a fast electron from the nucleus. Inside the nucleus a neutron changes into a proton and an electron: $$\text{neutron} \rightarrow \text{proton} + \text{electron}$$ After β-decay, mass number stays the same and atomic number increases by 1 (because there is one more proton).
• Gamma (γ): a high-energy electromagnetic wave. The nucleus loses extra energy. Mass number and atomic number do not change.

Decay changes written with nuclide notation

General forms:

Alpha: $${}^{A}_{Z}X \;\rightarrow\; {}^{A-4}_{Z-2}Y + {}^{4}_{2}\alpha$$

Beta (β⁻): $${}^{A}_{Z}X \;\rightarrow\; {}^{A}_{Z+1}Y + \beta^{-}$$

Gamma: $${}^{A}_{Z}X^{*} \;\rightarrow\; {}^{A}_{Z}X + \gamma$$ (the * shows an excited nucleus)

Key ideas to remember

• Decay is spontaneous and random. We cannot speed it up with heat, pressure, or light.
• After α or β decay, the atom changes to a different element (atomic number changes). After γ, it is the same element but with less energy.
• Alpha is strongly ionising but not very penetrating; beta is medium; gamma is weakly ionising but very penetrating.

Common misconception: The β⁻ electron comes from a neutron changing in the nucleus, not from the atom’s electron shells.