Specifically, mercury (Hg) is the only metal that is liquid at room temperature. It belongs to the transition metals, but is not a member of any other specific family.
While the alkali metals are not liquid at room temperature, they have very low melting points. This is because their atoms have only one valence electron, which is easily lost to form a positive ion. This makes the bonds between the atoms weak, resulting in low melting and boiling points.
