Standard free energy of a substance = f(T, P, state of substance, concentration,…). The Standard free energy change (ΔG⁰) corresponds to reactant’s and product’s free energies in their Standard states.
(ΔG⁰) = G⁰products – G⁰reactants
The Standard free energy of formation (ΔGf⁰) is the free energy change for a reaction in which a substance in its Standard state is formed from its elements in their most stable forms in their Standard states. Free energies of formation of the elements in their most stable forms at 1 atm pressure is zero.