Question

A certain negative ion $X^{2-}$ has in its nucleus $18$ neutrons and $18$ electrons in its extra nuclear structure. What is the mass number of $X$?

• A. $34$
• B. $32$
• C. $36$
• D. $39$

Answer 1

The correct option is A $34$

Number of electrons in $X^{2-}$ ion $=18$

Number of electrons in $X$ atom $=18-2=16$

Mass number $=$ number of electrons in neutral atom $+$ number of neutrons

$=16+18=34$