Explain the effects on the atomic (proton) number and mass (nucleon)…

The idea

Each type of radioactive decay changes the nucleus in a characteristic way, and you can predict the new atomic (proton) number and mass (nucleon) number by tracking the particles emitted. In alpha (α) decay the nucleus emits two protons and two neutrons, so the atomic number falls by 2 and the mass number falls by 4. In beta-minus (β⁻) decay a neutron becomes a proton and an electron is emitted, so the atomic number rises by 1 while the mass number is unchanged. In beta-plus (β⁺) decay a proton becomes a neutron and a positron is emitted, so the atomic number falls by 1 while the mass number is unchanged. Gamma (γ) emission releases energy only, so neither number changes. Neutron emission removes one neutron, so the mass number falls by 1 and the atomic number is unchanged. Writing a balanced nuclear equation for each case makes the changes easy to check.

What earns the marks

• States that α decay decreases the atomic number by 2 and the mass number by 4.
• States that β⁻ decay increases the atomic number by 1 and leaves the mass number unchanged.
• States that β⁺ decay decreases the atomic number by 1 and leaves the mass number unchanged.
• States that γ emission changes neither the atomic number nor the mass number.
• States that neutron emission decreases the mass number by 1 and leaves the atomic number unchanged.
• Balances nuclear equations to confirm the changes in atomic and mass number.

Common mistakes and how to put them right

• Treats γ emission as changing the atomic number.

  Put it right: Gamma emission releases energy only and changes neither number.

• Gives β⁻ decay as decreasing the atomic number.

  Put it right: Correct it to an increase of 1 because a neutron becomes a proton.

• Says neutron emission changes the atomic number.

  Put it right: State that only the mass number falls by 1.

Exam tips

• Write the nuclear equation and check that mass numbers and charge totals balance on both sides.
• Learn the change in atomic and mass number for each decay type as a short table.
• Read the question carefully to see whether it asks about atomic number, mass number or both.