How does the melting point of alkali metals change down the group?

The melting point of alkali metals decreases as you move down the group.

Alkali metals belong to Group 1 of the periodic table, which includes lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). One of the key trends in this group is the decrease in melting point as you move down the group, from lithium to francium.

This trend can be explained by the metallic bonding in alkali metals. Metallic bonding involves the sharing of free electrons among a lattice of positive ions. The strength of the metallic bond is determined by the charge of the ions and the size of the ion. In alkali metals, the charge of the ions is the same, +1, but the size of the ions increases down the group. This is because each successive element in the group has an extra electron shell.

As the size of the ion increases, the outermost electrons are further away from the positive nucleus and are less attracted to it. This means that the metallic bond becomes weaker as the distance between the nucleus and the outermost electrons increases. As a result, less energy is required to break the bonds and melt the metal, so the melting point decreases.

For example, the melting point of lithium is 180.5 degrees Celsius, while the melting point of sodium is 97.8 degrees Celsius. This trend continues down the group, with the melting point of potassium at 63.5 degrees Celsius, rubidium at 39.3 degrees Celsius, and caesium at 28.5 degrees Celsius. Francium, the heaviest alkali metal, is radioactive and has a very short half-life, so its melting point is difficult to measure, but it is expected to follow the same trend.

In summary, the melting point of alkali metals decreases down the group due to the increasing size of the ions, which weakens the metallic bond and reduces the amount of energy required to break the bonds and melt the metal.