How do the densities of alkali metals vary?

The densities of alkali metals generally increase as you move down the group in the periodic table.

Alkali metals are found in Group 1 of the periodic table. They include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). These metals are known for their low densities compared to other metals. However, the densities of these metals do not remain constant across the group, but rather, they generally increase as you move down the group.

Starting with lithium at the top of the group, it has the lowest density of all the alkali metals, at just 0.534 g/cm³. Sodium, the next alkali metal, has a slightly higher density of 0.97 g/cm³. Potassium has a density of 0.89 g/cm³, which is actually lower than sodium. This is an exception to the general trend, but after potassium, the densities of the alkali metals continue to increase. Rubidium has a density of 1.53 g/cm³, and caesium has a density of 1.93 g/cm³. Francium, the heaviest alkali metal, is radioactive and only exists in very small amounts, so its density is not well-known, but it is expected to follow the general trend and have a higher density than caesium.

The increase in density down the group can be explained by the increase in atomic mass. As you move down the group, each alkali metal has more protons and neutrons in its nucleus, making it heavier. Although the atoms also get larger due to the addition of electron shells, this increase in size is not enough to offset the increase in mass, so the overall density increases.