Why are individual hydrogen atoms more reactive than hydrogen molecules?

Individual hydrogen atoms are significantly more reactive than hydrogen molecules because of the energy required to break the stable bond within the molecule.

Molecular hydrogen ($H_2$) is a diatomic molecule held together by a strong chemical bond that requires a high dissociation energy of 104 kcal/mole to break. At room temperature, this high energy requirement results in negligible reaction rates, giving molecular hydrogen an appearance of inertness.1

In contrast, atomic hydrogen is highly reactive because it does not require this initial energy input to break a bond. Once generated—for example, through high-temperature contact with a filament or electric discharge—atomic hydrogen readily combines with most elements to form hydrides and acts as a powerful reducing agent. If these atoms are not consumed in a reaction, they will naturally recombine to form the more stable molecular state, a process that releases energy.1