So beta decay does not cause the mass of the nucleus to change but does…

The idea

In beta decay, a neutron in the nucleus changes into a proton and emits a beta particle, which is a fast-moving electron. The electron has negligible mass compared with nucleons, so the mass number stays the same. However, the nucleus gains one proton, so its proton number increases by 1 and its charge increases by 1 unit. For example, carbon-14 decays to nitrogen-14: ¹⁴₆C → ⁰₋₁e + ¹⁴₇N. The beta particle is written with mass number 0 and charge 1⁻. Because the proton number changes, the nucleus becomes a different element. Beta emission is common in nuclei with too many neutrons and is stopped by a few millimetres of aluminium.

What earns the marks

• A beta particle is a fast-moving electron emitted from the nucleus.
• In beta decay, a neutron changes into a proton and an electron.
• The mass number does not change because a neutron is replaced by a proton, so the number of nucleons stays the same.
• The proton number increases by 1, so the nuclear charge increases by 1 unit.
• The beta particle has mass number 0 and charge 1⁻ in a nuclear equation.
• The nucleus becomes a different element because its proton number increases by 1.

Common mistakes and how to put them right

• Saying the mass number increases by 1.

  Put it right by noting a neutron is replaced by a proton, so the total number of nucleons is unchanged.

• Thinking the charge decreases because an electron is emitted.

  Put it right by tracking the proton number, which increases by 1.

• Writing the beta particle as ⁰₊₁e.

  Put it right by using ⁰₋₁e, since the electron has charge 1⁻.

Exam tips

• Check that the mass number is the same on both sides of a beta decay equation.
• Show the proton number increasing by 1 from reactant to product.
• Remember that beta particles are electrons, so their charge is 1⁻ and their mass number is 0.