This question already has an answer here:
Hydrogen is in the first column of the periodic table. At STP it is a diatomic gas. When cooled and/or under pressure it forms a crystalline solid of di-hydrogen molecules. Calculations indicate that under high enough pressure it may form a monatomic basis metallic crystal, and scientists are actively looking to find evidence of this. Results so far seem inconclusive.
However, lithium is also a first column element. But its solid is a monoatomic basis metal and NOT a dilithium crystal as we would expect based on hydrogen. Why not?