Question

The electronic configuration of an ion $M^{2+}$ is $2,8,14$. If its mass number is $56$, then the number of neutrons in its nucleus is:

• A. $30$
• B. $32$
• C. $34$
• D. $42$

Answer 1

The correct option is A $30$

Given-

Metal (M) lose 2 electron to form $M^{2+}$

$M^{2+}=2,8,14$

Hence, electronic configuration of $M=2,8,16$

So, the total number of electrons in $M$ will be $=26$

Atomic number $\left(Z\right)=n_p=26$,

Mass number (given)$=n_p+n_n=56$

$\therefore$ Number of neutrons $n_p=A-Z=56-26=30$

Hence, option $A$ is the correct answer.