IGCSE Physics / Atoms and radioactivity
Nuclear equations and decay revision notes
Edexcel International GCSE Physics (4PH1), chapter 22: Atoms and radioactivity, specification point 7.7, 7.8. Free lesson.
Watch the video lesson · IGCSE Physics past papers
Nuclear equations show how a nucleus changes when it emits radiation. Balancing the mass numbers and atomic numbers lets you identify the new nucleus and the element it forms.
How each emission changes the nucleus
Track the mass number (the top number) and the atomic number (the bottom number) on each side of a nuclear equation. An alpha or beta emission changes the atomic number, so it makes a new element. Alpha emission gives out two protons and two neutrons. The mass number decreases by 4 and the atomic number decreases by 2. An alpha particle is written as ⁴₂He. Beta emission happens when a neutron turns into a proton and an electron. The electron leaves the nucleus, written as ⁰₋₁e. The mass number stays the same and the atomic number increases by 1. Gamma emission is energy,…
How to balance an alpha decay equation
The total mass number and total atomic number must each be the same on both sides. Subtract the numbers carried away by the alpha particle to find the numbers for the new nucleus.
How to balance a beta decay equation
The beta particle has 0 as its mass number and −1 as its atomic number. The top number therefore stays the same. For the bottom numbers to balance, the new nucleus has an atomic number one greater than the original.
Check both rows in a decay question
Use the same balancing rule for a question that asks only for the new nucleus. In alpha decay, subtract 4 from the mass number and 2 from the atomic number. For ²²⁶₈₈Ra decaying to radon by alpha emission, the radon nucleus has mass number 226 − 4 = 222 and atomic number 88 − 2 = 86. The answer is ²²²₈₆Rn. Both rows balance when the alpha particle, ⁴₂He, is included.