KCET · Chemistry · s Block Elements
For alkali metals, which one of the following trends is incorrect?
- A Hydration energy: \(\mathrm{Li}>\mathrm{Na}>\mathrm{K}>\mathrm{Rb}\)
- B Ionization energy : \(\mathrm{Li}>\mathrm{Na}>\mathrm{K}>\mathrm{Rb}\)
- C Density : \(\mathrm{Li} < \mathrm{Na} < \mathrm{K} < \mathrm{Rb}\)
- D Atomic size : Li \( < \mathrm{Na} < \mathrm{K} < \mathrm{Rb}\)
Answer & Solution
Correct Answer
(C) Density : \(\mathrm{Li} < \mathrm{Na} < \mathrm{K} < \mathrm{Rb}\)
Step-by-step Solution
Detailed explanation
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 \(\mathrm{Na}(86 \mathrm{pm})\) to \(\mathrm{K}(227 \mathrm{pm})\). Thus, the correct order of density is
\[
\mathrm{Li} < \mathrm{K} < \mathrm{Na} < \mathrm{Rb}
\]
\[
\mathrm{Li} < \mathrm{K} < \mathrm{Na} < \mathrm{Rb}
\]
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