What Happens to a Nucleus During Alpha, Beta, and Gamma Decay?

During alpha decay, the nucleus loses two protons and two neutrons; during beta decay, a neutron and proton are interconverted; during gamma decay, the nucleus loses energy without changing its proton or nucleon numbers. These changes are part of IB Physics subtopic E.3 Radioactive decay and apply at both SL and HL.

The Nuclear Changes

A nucleus is represented as $^{A}_{Z}\text{X}$, where $A$ is the nucleon number and $Z$ is the proton number. Nuclear equations must conserve total nucleon number, electric charge, and energy.

The emitted radiation carries away energy and momentum. In beta decay, the neutrino or antineutrino accounts for energy and momentum carried away during the conversion. Because $Z$ determines the element, alpha and beta decay produce a different element; gamma decay does not. For each equation, the upper and lower numbers balance on both sides.

For example, alpha decay can be written as

$$^{238}_{92}\text{U}\rightarrow{}^{234}_{90}\text{Th}+{}^{4}_{2}\text{He}.$$

Beta-minus decay changes the element because the proton number increases:

$$^{14}_{6}\text{C}\rightarrow{}^{14}_{7}\text{N}+e^-+\bar{\nu}_e.$$

A common misconception is that the beta particle was already inside the nucleus. It is instead created during the neutron-proton conversion. Gamma emission often follows alpha or beta decay when the daughter nucleus remains excited.

Exam Technique

In an IB determine question, balance both $A$ and $Z$ to identify the daughter nuclide or missing particle. Do not say gamma decay changes the element, and include the neutrino or antineutrino in beta-decay equations when required.