Atomic hydrogen is very reactive whereas molecular hydrogen is stable. This can be understood by the dissociation enthalpy of H₂ which is very high (435.88 kJ mol^–1).

So, from the dissociation enthalpy of H-H bond it is clear that H₂ bond will not easily break and it is relatively inert at room temperature. Thus it will only combine with few elements.

On the other hand atomic hydrogen being a single species can combine with other elements easily at room temperature.