Four ions $W$ , $X$ , $Y$ and $Z$ are the ions $O X^{2 -}$ , $F X^{-}$ , $Na X^{+}$ and $Mg X^{2 +}$ in some order. Their 6-coordinate radii are $W$ 72 pm, $X$ 140 pm, $Y$ 102 pm and $Z$ 133 pm. The sum of the atomic numbers of $W$ and $Y$ is:

Answer: B

For isoelectronic ions radius falls as

Z

rises. The order

X (140) > Z (133) > Y (102) > W (72)

matches

O X^{2 -}, F X^{-}, Na X^{+}, Mg X^{2 +}

. So

W

is Mg (

12

) and

Y

is Na (

11

); the sum is

23