The density of alkali metals is in the order

1. Periodic Trends

Density depends on both atomic mass and atomic volume according to the relation:

$$Density=Mass/Volume$$

As we move down the alkali metal group (Group 1):

• Both atomic mass and atomic volume increase.
• The increase in atomic mass is usually much more significant than the increase in volume, causing density to generally increase down the group.

2. The Sodium (Na) and Potassium (K) Anomaly

There is a well-known exception between Potassium (K) and Sodium (Na). When moving from Na to K, there is a large, sudden increase in atomic volume due to the filling of empty 3d orbitals in the shell layer.

As a result, the volume increase outpaces the mass increase, making Potassium less dense than Sodium.

3. Actual Density Values (g/cm³)

4. Correct Order

Arranging them from lowest to highest density based on empirical values: