JEE Main · 2019 · 09 Jan 2019 · Shift 1 · s-Block Elements

Among the alkaline earth nitrates, the tendency to crystallise with water of hydration falls as the cation grows, because a larger ion has lower charge density and binds water less strongly. $\mathrm{Ba}{}^{2+}$ is the biggest of the group given, so $\mathrm{Ba}(\mathrm{NO}{}_3){}_2$ crystallises anhydrous while the smaller $\mathrm{Mg}{}^{2+}$, $\mathrm{Ca}{}^{2+}$ and $\mathrm{Sr}{}^{2+}$ nitrates pick up water of crystallisation.