Question

A nucleus decays by ${\beta }^{+}$ - emission followed by a $\gamma$ -emission. If the atomic and mass numbers of the parent nucleus are $Z$ and $A$ respectively, the corresponding numbers for the daughter nucleus are respectively :

• A. $Z-1$ and $A-1$
• B. $Z+1$ and $A$
• C. $Z-1$ and $A$
• D. $Z+1$ and $A-1$

Answer 1

Answer: (C)

$\beta$ -rays are fast moving electrons.
When there is an excess of protons in the nucleus and it is not energetically possible to emit an $\alpha$ -particle, ${\beta }^{+}$ decay occurs. Resulting in reducing atomic numbers by $1.$ New atomic number $=Z-1$, mass number $=A.$ Gamma ray emission occurs with ${\beta }^{\prime }$ emission. Since gamma rays have no charge or mass their emission doesnot change the chemical composition of the atom.
Hence, atomic number $=Z-1$, mass number $=A$