How does beta decay change an atom’s atomic number?

In beta decay, an unstable atomic nucleus changes its atomic number by one unit while the mass number remains the same. The specific change depends on the type of decay process:1

• Negative beta decay (${\beta }^{-}$): A neutron in the nucleus transforms into a proton, causing the atomic number of the daughter nucleus to increase by one.1
• Positron emission (${\beta }^{+}$): A proton in the parent nucleus transforms into a neutron, causing the atomic number to decrease by one.1
• Electron capture: An orbital electron combines with a nuclear proton to form a neutron, which also results in the atomic number decreasing by one.1

These processes are governed by the weak force, which allows for the transmutation of nucleons between species. Because the atomic number defines the chemical element, these transformations result in the atom becoming a different element.231