JEE Main 2023 · 12 April · morning · Single correct · +4 / −1
The density of alkali metals is in the order :
- (a) Na<K<Cs<Rb
- (b) K<Na<Rb<Cs
- (c) K<Cs<Na<Rb
- (d) Na<Rb<K<Cs
Answer and solution
Answer (b) K<Na<Rb<Cs
Solution
Chemistry ma’am · 3 steps
Density mostly rises down the group, but potassium breaks the pattern and sits below sodium.
- General trend. Density is mass divided by volume. Going down group 1, atomic mass rises faster than atomic volume, so density usually increases.
Metal Density (g cm⁻³) Na 0.97 K 0.86, lower than Na Rb 1.53 Cs 1.90, the highest - The potassium exception. From Na to K the atom grows a lot, because a whole new shell is added. That jump in volume outweighs the gain in mass, so K ends up less dense than Na.
- So. The order is K<Na<Rb<Cs, which is option (b).
The idea to keep. For alkali metals, density increases down the group except that K is lighter than Na. Remember it as K < Na, then Rb < Cs at the top.
Answer from the NTA final answer key. Solution by PrepWiser.