A 23892U nucleus decays in two stages to a 23491Pa nucleus. What was emitted in these two stages?

Question 4

A ²³⁸₉₂U nucleus decays in two stages to a ²³⁴₉₁Pa nucleus.

What was emitted in these two stages?

A α + β          B α + γ          C β + β          D β + γ

Reference: Past Exam Paper – November 2008 Paper 1 Q40

Solution:

Answer: A.

During the decay: ²³⁸₉₂U        - - >     …        - - >  ²³⁴₉₁Pa + ???

For any decay, the proton number and mass number must be conserved.

Proton number: 92 = 91 + x

Mass number: 238 = 234 + y

Thus, x = 92 – 91 = 1              and y = 238 – 234 = 4

3 types of radiation can be emitted during radioactive decay: alpha, beta (minus) and gamma. We know that alpha particle is ⁴₂α and beta particle is ⁰_-1β. Gamma is an EM wave and does not have a mass or proton number.

We need to identify which of the combinations provided in the multiple choice would give an overall proton number of +1 and overall mass number of 4.

Consider choice A:     [α and β]

Proton number = 2 + –1 = +1

Mass number = 4 + 0 = 4

So, A is correct