The standard enthalpy of formation $\Delta_f H^\circ$ of any element in its most stable form at the standard state is:

By convention, the standard enthalpy of formation of an element in its most stable allotropic form at the standard state (typically 1 bar, 298 K) is taken to be zero. So $\Delta_f H^\circ(\mathrm{O_2, g}) = 0$, $\Delta_f H^\circ(\mathrm{C, graphite}) = 0$, etc. This convention provides a reference relative to which the formation enthalpies of compounds are measured.