Beta Decay (AQA GCSE Physics)

A nucleus with 11 protons and 13 neutrons undergoes beta decay. It forms magnesium, which has the element symbol Mg.

Which is the correct isotope of magnesium formed during the decay?

Answer: D

Step 1: Calculate the mass number of the original nucleus

• The mass number is equal to the number of protons plus the number of neutrons

• The original nucleus has 11 protons and 13 neutrons

11 + 13 = 24

• The mass number of the original nucleus is 24

Step 2: Calculate the new atomic number

• During beta decay a neutron changes into a proton and an electron

• The electron is emitted as a beta particle

• The neutron has an atomic number of 0 and the proton has an atomic number of 1

• So the atomic number increases by 1

11 + 1 = 12

• The new nucleus has an atomic number of 12

Step 3: Calculate the new mass number

• Protons and neutrons both have a mass number of 1

• Changing a neutron to a proton will not affect the mass number

• The new nucleus has a mass number of 24 (the same as before)

There is a second form of beta decay during which a proton changes into a neutron. This is called beta-plus decay - you might come across it while revising, but you don't need to know about it for your exam. Only use the information here for your GCSE.