Question

Anion is larger in size than parent atom. Why?

Answer 1

The formation of anion takes place when an atom gains electrons. Example....
Br + 2e- ———› Br(2-)
(2-) is it's valency.
Now, when it gains electrons, there's no change in the composition of it's nucleus (that is, the number of nucleons or better to say protons). So, the Nuclear charge remains the same due to which the nuclear pull on the valence shell will also remain the same. But the valence shell gains electrons and stabilizes itself due to which the nuclear force can no longer pull it with the same force. This results in the increase in atomic radii of the ion (more specifically, anion).
Therefore, Anion is larger in size than the parent atom.

The opposite happens in the case of cation.

Thanks.
Hope this helps you..!!

Answer 2

Anion is larger than the parent atom because of the smaller effective nuclear charge of the anion.

What is anion?

• An anion is an ionic species with a negative charge. This is a sort of atom that has acquired electrons.
• Since it has more electrons than protons, anions have a negative charge.

Why anion is larger than the parent atom?

• Electrons help in determining the size of atoms and molecules.
• The surplus electrons of an anion repel each other and add to the physical size of the ion and we know that the size of an ion is governed by its electron cloud.
• This results in the smaller effective nuclear charge of the anion so, it has a large size.

Therefore, we can conclude that it's due to a very low effective nuclear charge of the anion that gives it large size.