Why is hydrogen gas made of H2 molecules under ordinary conditions?

Under ordinary conditions, hydrogen exists as a diatomic molecule, $H_2$, because this configuration allows the atoms to achieve a lower total energy than they would have as widely separated, individual atoms.12

This stability is explained by molecular orbital theory, which describes how the valence-shell orbitals of two hydrogen atoms interact:

• Bonding Interaction: When two hydrogen atoms approach, their $1s$ atomic orbitals overlap constructively. This creates a bonding molecular orbital (designated as $1\sigma$) where electron density is concentrated between the two nuclei.3
• Energy Lowering: Because the electrons in this bonding orbital are attracted to two nuclei simultaneously rather than one, their energy is significantly lower than when they are confined to an individual atom.13
• Electron Configuration: Following the building-up principle, the two available electrons occupy this lower-energy bonding orbital, resulting in a stable $1{\sigma }^2$ configuration.3

In contrast, if hydrogen atoms remained separate, they would possess higher energy. The formation of the $H_2$ molecule is thus a natural process driven by the system's tendency to reach a state of minimum energy.1