A radioactive nucleus is formed by β-decay. This nucleus then decays by α-emission. Which graph of proton number Z plotted against nucleon number N shows the β-decay followed by the α-emission?

Question 7

A radioactive nucleus is formed by β-decay. This nucleus then decays by α-emission.

Which graph of proton number Z plotted against nucleon number N shows the β-decay followed by the α-emission?

Reference: Past Exam Paper – June 2008 Paper 1 Q40

Solution:

Answer: B.

An alpha particle is ⁴₂He and a beta particle is ⁰_-1β.

Proton number Z and nucleon number N will be conserved.

Let the proton number of the initial nucleus be Z and its nucleon number be N.

During the β-decay (⁰_-1β),

the proton number of the new nucleus increases by one such that (Z+1) + (-1) = Z.

            the nucleon number remains the same.

In the graph, the change in horizontal and to the left.

During the alpha-emission (⁴₂He),

proton number of new nucleus decreases by 2 such that (Z-2) + 2 = Z.

            nucleon number decreases by 4 such that (N-4) + 4 = N