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. 36
• B. 34
• C. 32
• D. 39

Answer 1

The correct option is B 34

$X^{2-}$ has 18 $e^{-}$. Therefore Z = 16

Atomic mass = No. of $p^{+}$ + No. of $n^{\circ }$ = 16 + 18 = 34