Question

Which of the following has the largest size?
(A) $Na$
(B) $N{a^ + }$
(C) $Mg$
(D) $M{g^ + }$

Answer 1

Atomic size is the distance between the center of the nucleus and the outermost electron in the outer shell. Across a period from left to right there is decrease in atomic size with increase in nuclear charge of the element. Atomic size increases down the group because of the addition of extra shells.

Complete Step By Step Answer:
Atomic radius decreases across a period because valence electrons are being added to the same energy level at the same time the nucleus is increasing in protons. The increase in nuclear charge attracts the electrons more strongly, pulling them closer to the nucleus.
The size of $Na$ is bigger than $Mg$ due to increased nuclear charge because of which force of attraction between the nucleus and valence electrons increases therefore atomic size decreases and Size of cation is lower than neutral atom because as a particular atom loses electron, no of proton become more than no of electron and attraction forces increases so radius decreases.
Exit of electrons from outermost orbit results in formation of cation. In this process, removal of the outermost orbit increases the effective nuclear charge on remaining electrons. Hence, the size of a cation is smaller than that of a neutral atom. Hence, the size of an anion is larger than that of a neutral atom.
So, the correct answer is (A) $Na$ .

Note:
The size of cation is smaller and cation bigger in size than their corresponding neutral atom because Anion is formed by accepting electrons, so the intermolecular force acts on more electrons which doesn't shrink the size of an atom much. All this explanation is theoretical. Due to increased nuclear charge, cation is smaller, due to decreased nuclear charge, anion is larger than that of parent neutral atoms.