Why do alkali metals have low melting points?

Alkali metals have low melting points due to their single electron in the outermost shell and weak metallic bonding.

Alkali metals, which include lithium, sodium, potassium, rubidium, caesium, and francium, are found in Group 1 of the Periodic Table. These metals are characterised by their single electron in the outermost shell. This single electron is relatively far from the nucleus and is therefore not strongly attracted to it. This results in a weak hold by the nucleus on the electron, which in turn leads to weak metallic bonding.

Metallic bonding is the force of attraction between positive metal ions and the sea of delocalised electrons surrounding them. In alkali metals, this bonding is particularly weak due to the single electron in the outermost shell. This weak bonding means that less energy is required to break the bonds and change the metal from a solid to a liquid, resulting in a low melting point.

Furthermore, as you move down Group 1 of the Periodic Table, the melting points of the alkali metals decrease. This is because the outermost electron gets further from the nucleus and is less attracted to it, making the metallic bonding even weaker. For example, lithium has a melting point of 180°C, while sodium melts at 98°C, and potassium at just 63°C. This trend continues with the other alkali metals.

In summary, the low melting points of alkali metals can be attributed to their single electron in the outermost shell and the resulting weak metallic bonding. This makes alkali metals distinct from other metals, which typically have higher melting points due to stronger metallic bonding.