Radioactive Decay

Certain nuclei are unstable. They have too many protons, too many neutrons, or simply too much energy, and they cannot remain in their current state indefinitely. To move towards stability, these nuclei undergo radioactive decay — a spontaneous process in which the nucleus emits radiation and transforms into a different nuclear configuration.

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Types of Radioactive Decay

There are four principal types of radioactive decay you need to know for Edexcel A-Level Physics. Each involves a different emission and a different change to the nucleus.

Alpha (α) Decay

An alpha particle is a helium-4 nucleus: two protons and two neutrons bound together (⁴₂He). When a heavy, proton-rich nucleus undergoes alpha decay, it ejects an alpha particle.

Changes to the nucleus:

• Nucleon number A decreases by 4
• Proton number Z decreases by 2

General equation: ᴬ_Z X → ᴬ⁻⁴_(Z-2) Y + ⁴₂α

Example: ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂α

Alpha decay typically occurs in heavy nuclei (Z > 82) where the nucleus is too large for the strong nuclear force to hold it together against electromagnetic repulsion.

Beta-Minus (β⁻) Decay