Q. For alkali metals, which one of the following trends is incorrect?
A) Hydration energy : Li > Na > K > Rb
B) Ionisation energy : Li > Na > K > Rb
C) Density : Li < Na < K < Rb
D) Atomic size : Li < Na < K < Rb
Explanation
(c)
In general, density increases on moving downward in a group but density of potassium (K) is lesser than that of the sodium (Na). This is because of the abnormal increase in atomic size on moving from Na (86 pm) to K (227 pm).
Thus, the correct order of density is
$Li < K < Na < Rb$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.