Answer
$N^{3-}, O^{2-}, F^{-}$ is in the decreasing order of the atomic radius.
Work Step by Step
$N^{3-}, O^{2-}, F^{-}$ is in the decreasing order of the atomic radius.
(a) $Be, Mg, Ba$.
Beryllium, Magnesium, and Barium belong to group 2A. Be, Mg, and Ba are the first, second, and fifth members respectively. The atomic radius increases on moving down the group.
(b) $N^{3-}, O^{2-}, F^{-}$.
These are isoelectronic species with 10 electrons, but the nuclear charges of
$N^{3-}, O^{2-}, F^{-}$ are 7, 8, and 9 respectively. As the nuclear charge increases atomic radius decreases in the above isoelectronic species.
(c) $Tl^{3+}, Tl^{2+}, Tl^{+}$.
The atomic number of Thallium is 81. $Tl^{3+}, Tl^{2+}, and Tl^{+}$ are formed by the removal of 3,2, and 1 electron from the Thallium atom respectively. As the nuclear charge remains the same, the ion with the fewest electrons is attracted more effectively.
