Valence bond theory

The chemist Charles Rugeley Bury suggested in 1921 that eight and eighteen electrons in a shell form stable configurations.

[2] In 1916, Kossel put forth his theory of the ionic chemical bond (octet rule), also independently advanced in the same year by Gilbert N.

Specifically, Walter Heitler determined how to use Schrödinger's wave equation (1926) to show how two hydrogen atom wavefunctions join together, with plus, minus, and exchange terms, to form a covalent bond.

Building on this article, Pauling's 1939 textbook: On the Nature of the Chemical Bond would become what some have called the bible of modern chemistry.

However, the later edition in 1959 failed to adequately address the problems that appeared to be better understood by molecular orbital theory.

For example, the carbon in methane (CH4) undergoes sp3 hybridization to form four equivalent orbitals, resulting in a tetrahedral shape.

Different types of hybridization, such as sp, sp2, and sp3, correspond to specific molecular geometries (linear, trigonal planar, and tetrahedral), influencing the bond angles observed in molecules.

Hybrid orbitals provide additional directionality to sigma bonds, accurately explaining molecular geometries.

[10] Valence bond theory complements molecular orbital theory, which does not adhere to the valence bond idea that electron pairs are localized between two specific atoms in a molecule but that they are distributed in sets of molecular orbitals which can extend over the entire molecule.

In particular, MO theory can effectively account for paramagnetism arising from unpaired electrons, whereas VBT struggles.

[11] Valence bond theory views aromatic properties of molecules as due to spin coupling of the π orbitals.

[12][13][14][15] This is essentially still the old idea of resonance between Friedrich August Kekulé von Stradonitz and James Dewar structures.

For example, in the case of the F2 molecule, the F−F bond is formed by the overlap of pz orbitals of the two F atoms, each containing an unpaired electron.