The Chemical Bond: When Atom Size Instead of Electronegativity Difference Determines Trend in Bond Strength

We have quantum chemically analyzed element−element bonds of archetypal HnX−YHn molecules (X, Y=C, N, O, F, Si, P, S, Cl, Br, I), using density functional theory. One purpose is to obtain a set of consistent homolytic bond dissociation energies (BDE) for establishing accurate trends across the periodic table. The main objective is to elucidate the underlying physical factors behind these chemical bonding trends. On one hand, we confirm that, along a period (e. g., from C−C to C−F), bonds strengthen because the electronegativity difference across the bond increases. But, down a period, our findings constitute a paradigm shift. From C−F to C−I, for example, bonds do become weaker, however, not because of the decreasing electronegativity difference. Instead, we show that the effective atom size (via steric Pauli repulsion) is the causal factor behind bond weakening in this series, and behind the weakening in orbital interactions at the equilibrium distance. We discuss the actual bonding mechanism and the importance of analyzing this mechanism as a function of the bond distance. Paradigm Switch! Chemical bonds in general become stronger as the electronegativity difference across the bond increases. This correlation has hitherto been considered causal but, here, we show it is not in some remarkable cases. For example, carbon−halogen bonds become weaker from C−F to C−I, despite stronger (not weaker) orbital interactions as the bond overlap increases, because of stronger Pauli repulsion with the larger, more electron‐rich halogen.