The standard enthalpy of formation of O2(g) is defined as exactly 0 kJ/mol, while that of O3(g) is about +142 kJ/mol. Why the difference?

Core Concept

ΔH°f is set to zero only for an element in its standard state, the most stable form it takes at 1 atm and the stated temperature. Dioxygen is that reference form for oxygen, so its value is defined as zero. Ozone is a less stable allotrope, and forming it from O2 requires an input of energy, which is why its value is positive. The nonzero entry reflects a real energetic difference between allotropes.