JEE Main 2022 · 25 July · morning · Single correct · +4 / −1
Choose the correct order of density of the alkali metals :
- (a) Li < K < Na < Rb < Cs
- (b) Li < Na < K < Rb < Cs
- (c) Cs < Rb < K < Na < Li
- (d) Li < Na < K < Cs < Rb
Answer and solution
Answer (a) Li < K < Na < Rb < Cs
Solution
Chemistry ma’am · 3 steps
Density rises down the group, with one famous exception: potassium is lighter than sodium.
- General trend. Going down the group, both atomic mass and atomic size increase. Mass usually grows faster than volume, so density generally increases from Li to Cs.
- The exception. From Na to K, the atom gets much larger, because a whole new shell is added and the metallic bonding becomes weaker. The volume increase outweighs the mass increase, so K is less dense than Na.
Metal Density (g cm⁻³) Li 0.53 Na 0.97 K 0.86 Rb 1.53 Cs 1.87 - So. Arranging these values in increasing order gives Li < K < Na < Rb < Cs, which is option (a).
The idea to keep. For alkali metal density, remember the trend plus the one kink: it rises down the group, except that K is lighter than Na. Lithium is the least dense of all.
Answer from the NTA final answer key. Solution by PrepWiser.