Why does hydrogen occur in a diatomic form rather than in a monoatomic form under normal conditions?
The ionization enthalpy of hydrogen atom is very high (1312 kJ mol–1). Hence, it is very hard to remove its only electron. As a result, its tendency to exist in the monoatomic form is rather low. Instead, hydrogen forms a covalent bond with another hydrogen atom and exists as a diatomic (H2) molecule.
In diatomic form, the K-shell of hydrogen is complete (1s2) and so it is quite stable.