p-Block Elements: PYQ Walkthrough (10)

Question

Arrange the following in increasing order of acidic strength of their oxides: $N_2O_5, P_2O_5, As_2O_5, Sb_2O_5, Bi_2O_5$. Justify your answer.

A NEET 2022 PYQ on Group 15 trends.

Solution — Step by Step

Final answer: $Bi_2O_5 < Sb_2O_5 < As_2O_5 < P_2O_5 < N_2O_5$.

Why This Works

Acidic oxides react with water to give acids and with bases to give salts. The more electronegative the element, the more it pulls electron density away from the O-H bond in the corresponding oxoacid, releasing $H^+$ more easily.

Down the group, atomic size increases and electronegativity decreases. Larger, less electronegative elements form more ionic bonds with oxygen — and ionic oxides tend to be basic.

Alternative Method

Use Fajans’ rules. Smaller cation = more covalent oxide = more acidic. $N^{5+}$ is tiny and highly polarising, so $N_2O_5$ is strongly acidic. $Bi^{5+}$ is large and weakly polarising, so $Bi_2O_5$ is basic.

Common Mistake

Ranking by molecular weight or boiling point instead of by elemental electronegativity. Acidity is a chemical property — it correlates with electronegativity and metallic/non-metallic character, not with size or mass.