Dihydrogen is quite stable and dissociates into hydrogen atoms only when heated at about 2000 K. $\mathrm{H}_{2(g)}$ $\xrightarrow{2000\,\mathrm{K}}$ $\mathrm{H}_{(g)}+\mathrm{H}_{(g)}$ Above 2000 K, the dissociation of hydrogen into atoms is only 0.081% which increases to 95.5% at 5000 K. Its bond dissociation energy is very high. $\mathrm{H}_2$ → H + H ; ΔH = 435.9 $\mathrm{kJ\,mol^{-1}}$ Due to its high bond dissociation energy it is not very reactive.