Towards a unified theory of bonding

Chemical bonds are one of the first ideas we learn about in chemistry, where foundational concepts including electron sharing (covalency), transfer (ionicity), and delocalisation (metallicity) are used to understand chemical properties such as bond strengths, lattice energies, 3D structures, solubility and conductivity.

While on the surface these ideas seem well-established, an ongoing debate has been rumbling behind the scenes for over 100 years. With the discovery of the electron and the birth of quantum mechanics in the late 19th and early 20th century came questions about the wave-like nature of these quantum particles. Two camps formed, each with their own opinions on the origins of covalent bonding.

The first, initiated by John Slater, favoured the explanation that by increasing the density of electrons between positively charged nuclei, net electrostatic attraction holds atoms together. It is this explanation that is often first taught to chemistry students. The second camp formed around Hans Hellman’s quantum explanation, which hinges upon the wave-like behaviour of electrons: when an electron delocalises to be shared between two atoms in a chemical bond, its wavelength increases. Since kinetic energy depends inversely on wavelength, a longer wavelength of light corresponds to a lower energy photon, and the same is true of electrons. This energy lowering results in a more stable molecule, which we associate with bond formation.