Question

The density of the alkali metals is in the order:

• A. K<Na<Rb<Cs
• B. Na<K<Cs<Rb
• C. K<Cs<Na<Rb
• D. NavRb<K<Cs

Answer 1

The Correct Option is A

The density of alkali metals is determined by their atomic structure and mass. Alkali metals belong to Group 1 of the periodic table, and as we move down the group, the atomic size and atomic mass of the metals increase. However, their metallic bonding does not get significantly stronger, leading to an interesting trend in density.

Let's analyze the densities of these alkali metals: 

• Lithium (Li): 0.534 g/cm³
• Sodium (Na): 0.971 g/cm³
• Potassium (K): 0.862 g/cm³
• Rubidium (Rb): 1.532 g/cm³
• Cesium (Cs): 1.873 g/cm³

On examining the data above, we can conclude the following order of densities among the given alkali metals:

\(\text{K} < \text{Na} < \text{Rb} < \text{Cs}\)

This is due to the fact that as you move down the group, even though the atomic mass increases, the increase in volume does not compensate for the increase in mass, leading to a general increase in density. Potassium (K), being lighter than Sodium (Na), starts with a lower density, but afterwards, the trend follows the expected increase.

Thus, the correct answer is K < Na < Rb < Cs.