Question

A radioactive substance has an average life of $5 hr$. In a time of 5 hours:

• A. Half of the active nuclei decay
• B. Less than half of the active nuclei decay
• C. More than half of the active nuclei decay
• D. All active nuclei decay

Answer 1

Answer: (C)

If $\lambda$ is the decay constant of a radioactive substance then average life $=\frac{1}{\lambda}$
Also half life $=\frac{0.693}{\lambda} $
$\Rightarrow 0.693 \times \lambda$
In single average life, more than $63 \%$ of radiactive nuclei decay.