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.
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