A Levels Physics

An isotope of thorium has a nucleon number of 232 and a proton number of 90. It decays to form
another isotope of thorium with a nucleon number of 228.
How many alpha particles and beta particles are emitted by a nucleus of thorium during this
decay?
alpha particles beta particles
A
4
1
1
2
D
2
1


Choice C is correct because:

Note: The only radioactive emission that can cause the nucleon number of a particle to change is alpha particle emission.

One alpha particle emitted will result in the nucleon number (of thorium) to change from 232 to 228.

General equation of the emission of an alpha particle: A X Z → A-2 Y Z-4 + 2 α 4

The equation for alpha particle emission of Thorium: 90 Th 232 → 88 X 228  + 2 α 4

Number of alpha particles emitted = 1

However, 88 X 228 is not an isotope of thorium as it does not have the same number of protons as thorium.

Note: Each β− particle emission causes the proton number to increase by 1 (a neutron changes into a proton). However, the nucleon number remains the same.

Two beta particles emitted will result in the proton number (of X) to change from 88 to 90. This will result in the formation of a thorium isotope.

General equation of beta particle emission: A X Z → A+1 Y Z + β− + ν̅  + energy

Two beta particle emission of nucleus X: 88 X 228 → 88 + 2 Th 228 + 2(β−) + 2(ν̅ ) + energy

Two beta particle emission of nucleus X: 88 X 228 → 90 Th 228 + 2(β−) + 2(ν̅ ) + energy

Number of beta particles emitted = 2

Choices A, B & D are incorrect because they do not correctly state the number of alpha particles emitted as 1 & the number of beta particles emitted as 2.

Radioactive Decay | 2014 - M/J - 9702/13 | Q39