Question

The mass number of an atom whose unipositive ion has $10$ electrons and $12$ neutrons is

• A. $22$
• B. $23$
• C. $21$
• D. $20$

Answer 1

The correct option is B

$23$

The explanation for the correct option: (B)
1. The mass number of an atom is the sum of the atomic number and the number of neutrons.
2. The number of electrons or protons represents the atomic number. Here the number of electrons present in the atom is $11$.
3. Thus the mass number is given by the formula:

$\begin{array}{rcl}& & \text{M=n+Z}\\ & =& 11+12\\ & =& 23\end{array}$

Hence, option (B) is correct- The mass number of an atom whose unipositive ion has $10$ electrons and $12$ neutrons is $23$.