Radioactive Decay Equations

This lesson covers how to write and balance nuclear decay equations for alpha, beta and gamma decay — as required by the Edexcel GCSE Physics specification (1PH0), Topic 6: Radioactivity. You need to understand how the atomic number and mass number change during each type of decay, and be able to write balanced nuclear equations.

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Nuclear Notation

Before writing decay equations, you need to understand nuclear notation. An atom or particle is written as:

ᴬ_Z X

Where:

• A = mass number (top) — total number of protons + neutrons
• Z = atomic number (bottom) — number of protons
• X = chemical symbol of the element

For example, uranium-238 is written as: ²³⁸₉₂U (mass number 238, atomic number 92).

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Alpha (α) Decay

In alpha decay, the nucleus emits an alpha particle — a helium-4 nucleus containing 2 protons and 2 neutrons.

What Happens to the Numbers?

Number	Change
Mass number (A)	Decreases by 4 (2 protons + 2 neutrons are lost)
Atomic number (Z)	Decreases by 2 (2 protons are lost)