When dG equals zero, eq. ( ) represents the equilibrium condition between atoms exchanging electrons within a molecule, or any other material. In such a case, the first right-hand side term of eq. ( ) corresponds to an electronegativity difference. To see why, we look at the example of a heteronuclear diatomic molecule AB. In such a molecule, the change in electron population of one atom (relative to the isolated atom) must perfectly equal that of the other atom, but with reversed sign, i.e., d〈n L 〉 = -d〈n k 〉. It then follows that ∑ 𝑞 ! 〈χ *,FGH 〉d〈n [ 〉 [ is exactly the difference in electronegativity between the two atoms in the molecule, 〈χ l,FGH 〉 -〈χ m,FGH 〉. Equation ( ) explains why differences in electronegativity between non-equivalent atoms (or fragments) are expected to persist at equilibrium. Atomic electronegativities (of the sort we investigate c.f., eqs. ( ) and ( )) can equalize only in special cases where contributions from hardness, core shifts and electrostatic repulsions cancel out. Equation (22) therefore formalizes the concept of electronegativity equilibration and explains why equalization of electronegativity is not strictly necessary.