Why is fluorine a gas, but lithium isn't?
A lithium atom has one valence electron, easily lost (shared), so it is connected to other atoms by a metallic bond. This is a  bit like the shell game where a pea (electron) is hidden under a walnut shell... the uncertainty of where it is at any instant implies, in a quantum sense, that all the atoms share it, and are held together. This bond is so strong that the next element, beryllium, has a boiling point of $\pu{3243 K}$.

Fluorine, on the other hand, has an almost complete shell and each atom forms a tight covalent bond with just one other fluorine atom. These molecules of $\ce{F2}$ keep their electrons to themselves and do not associate with each other.

The difference in bonding is like the difference between bricks mortared together in pairs, or mortared together in a three-dimensional structure, and is responsible for the difference in mp and bp.