Standard enthalpy of formation of an element in its standard state:

ΔH_f° for elements in standard state (most stable form at 298 K, 1 atm) = 0 by definition. e.g., ΔH_f°(O₂, g) = 0, ΔH_f°(C, graphite) = 0, ΔH_f°(H₂, g) = 0.