Which is greater in size ?

(a) An atom or a cation

(b) An atom or an anion

(c) Fe²⁺ or Fe³⁺

(d) Fluorine or oxygen

(a) An atom.

Reason — An atom is greater in size than a cation because cation is formed by the loss of electron(s), hence proton(s) are more than electron(s) in a cation. So electrons are strongly attracted by the nucleus and are pulled inward. Hence, the size decreases.

(b) An anion.

Reason — An anion is greater in size than atom because anion is formed by the gain of electron(s). Thus, the number of electron(s) are more than proton(s). The effective positive charge in the nucleus is less, so less inward pull is experienced. Hence, the size increases.

(c) Fe²⁺.

Reason — Fe²⁺ is greater in size than Fe³⁺ because Fe²⁺ has one more electron than Fe³⁺. So, nuclear pull is more in Fe³⁺ than Fe²⁺, thereby bringing the outermost shell closer to the nucleus. Hence, size decreases.

(d) Oxygen.

Reason — Oxygen is greater in size than fluorine. The position of oxygen in periodic table is before fluorine so, as we move from left to right in a period the outermost shell with more number of electrons is brought closer to the nucleus, decreasing the size of the atom.