Question

$\text{Assertion}$ : Electron capture occurs more often than positron emission in heavy elements.
$\text{Reason}$ : Heavy elements exhibit radioactivity.
Read the assertion and reason carefully to mark the correct option out of the options given below:

• A. Both assertion and reason are true and the reason is the correct explanation of the assertion.
• B. Both assertion and reason are true but reason is not the correct explanation of the assertion.
• C. Assertion is true but reason is false.
• D. Assertion is false but reason is true.

Answer 1

The correct option is B Both assertion and reason are true but reason is not the correct explanation of the assertion.

The positron emission is represented by,

$p\to n+e^{+}+\nu$

Electron capture occurs more often than positron emission in heavy elements.

This is because, the rest mass of proton(reactant) is less than that of neutron(product).

So, positron emission is energetically not allowed.

However, for electron capture, the electrons are readily available and energetically allowed.

Therefore, the assertion is true.

All those elements which are heavier than lead are radioactive. This is because in the nuclei of heavy atoms, $\text{Binding Energy}$ per unit $\text{Atomic mass}$ is low and hence is unstable.

So, Reason is also true, but it is not an explanation of assertion.

Hence, option $\left(\text{B}\right)$ is correct.

Why this Question? $\text{K}$ capture: It is a rare process which is found only in few nuclei. In this process, the nucleus captures one of the atomic electrons from the $\text{K}$ shell. A proton in the nucleus combines with this electron and converts itself into a neutron. A neutrino is also emitted in the process and is emitted from the nucleus.