Question

Which one of the following is the smallest in size?

• A.
• B.
• C.
• D.

Answer 1

The correct option is D

Element	$N^{3-}$	$O^{2-}$	$F^{-}$	$N{a}^{+}$
No. of electrons	10	10	10	10
No.of protons	7	8	9	11

It can be observed from the above table that all elements have the same number of electrons, i.e. 10 but they have different number of protons.
The same number of electrons suggest that all the elements will have the same number of shells in which electrons are filled.
In case of nitrogen 10 electrons get a pull towards the nucleus by 7 protons while in case of oxygen, fluorine and sodium the number of protons are more than that. More the number of protons, the more will be the pull towards the nucleus and smaller will be the size.
Hence, the correct order of size will be:
$N^{3-}$ > $O^{2-}$ > $F^{-}$ > $N{a}^{+}$