Enthalpies of formation define the amount of heat needed to produce a molecule from its constituent atoms. The standard enthalpy of formation (DH_f °) is the arbitrary reference point for enthalpy; these values are based on the standard state conditions of 1 mole and 1 atm.

The standard enthalpy of formation of any element in its most stable form is defined as zero. In terms of sign convention, negative numbers are used for exothermic reactions and positive numbers for endothermic reactions.

Lets examine the values for several forms of oxygen. Molecular (diatomic) oxygen is the standard elemental form. Oxygen atoms (O) and ozone (O₃) are higher energy forms as evidenced by the enthalpy of formation data in the table.