Molecules have characteristic shapes that are crucial for their function. The arrangement of various electron groups around the central atom dictates their molecular geometry. Electron pairs in the valence shell of a central atom will adopt an arrangement that minimizes repulsions between the electron pairs by maximizing the distance between them. The valence electrons form either bonding pairs, located primarily between bonded atoms, or lone pairs.
Two regions of electron density in a diatomic molecule are oriented linearly on opposite sides of the central atom to minimize repulsions, and three electron groups are arranged in a trigonal planar geometry. Four electron groups form a tetrahedron, five electron groups have trigonal bipyramidal geometry, and six groups are oriented octahedrally.
It is important to note that the electron-pair geometry around a central atom is not always the same as its molecular structure. The electron-pair geometry describes all the regions where electrons are located in a molecule, in bonds and lone pairs. The molecular structure describes the location of the atoms in a molecule, not the electrons. This means that the electron-pair geometry is the same as the molecular structure only when there are no lone electron pairs around the central atom.
A lone pair of electrons occupies a larger space than a bonding pair because a lone pair is bound to only one nucleus, but two nuclei share a bonding electron group. Because of this, lone pair–lone pair repulsions are greater than lone pair–bonding pair and bonding pair–bonding pair repulsions.
Part of this text is adapted from Openstax, Chemistry 2e, Section 7.6: Molecular Structure and Polarity.
Reference:
Flowers, P., Theopold, K., Langley, R., Robinson, W. R., Clark, M. A., Douglas, M., Choi, J. Section 7.6: Molecular Structures. In Chemistry 2e. OpenStax. Houston, TX (2019).
