Q.For alkali metals, which one of the following trends is incorrect ?
Step 1. Hydration energy (kJ mol⁻¹, magnitude): Li -506, Na -406, K -330, Rb -310 -- this falls steadily, so Li > Na > K > Rb is correct as stated in (a).
Step 2. First ionisation energy (kJ mol⁻¹): Li 520.2, Na 495.8, K 418.8, Rb 403.0 -- also falls steadily, so Li > Na > K > Rb is correct as stated in (b).
Step 3. Atomic radius (Å): Li 1.52, Na 1.86, K 2.27, Rb 2.48 -- rises steadily, so Li < Na < K < Rb is correct as stated in (d).
Step 4. Density (g cm⁻³): Li 0.54, Na 0.97, K 0.86, Rb 1.53 -- this does NOT rise smoothly: potassium (0.86) is actually lower than sodium (0.97), an anomaly caused by potassium's larger atomic volume increasing faster than its mass at that point in the group. So the claimed trend Li < Na < K < Rb is broken at K, making option (c) the incorrect trend.
(c) Density : Li < Na < K < Rb
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