Question

For alkali metals, which one of the following trends is INCORRECT?

• A. Ionization energy : $Li > Na > K > Rb$
• B. Hydration energy : $Li > Na > K > Rb$
• C. Atomic size : $Li < Na < K < Rb$
• D. Density : $Li < Na < K < Rb$

Answer 1

Answer: (D)

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

$Li < K < Na < Rb$