Molecular Geometry & Bonding
what the shape decides
Concept

Fluxionality — where it appears

The rapid interconversion of a molecule between equivalent structures. What a spectrum then sees is an average over the rearrangement, so the rigid point group of any one structure is the wrong group to use.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

Five sites are not five of a kind. The minimised arrangement of five points, with the two axial sites marked apart from the three equatorial ones. Their neighbour angles differ, so the two kinds of position are genuinely different places — which the shape's name does not convey.

Five sites are not alike

Every other common arrangement has one or two distinct angles. Five has three, because two of its positions are on an axis and three are round an equator — and a molecule built that way does something about it.

shape · VSEPR Three counts for each molecule, and the barrier between them. Four molecules, with the number of operations available to each: the point group, which is what a rigid rotation or reflection can do; the feasible group, which adds whatever a crossable barrier makes available; and the whole permutation-inversion group, which is every rearrangement of identical nuclei whether a molecule can reach it or not. The middle column is the one that describes a real spectrum, and it equals the first only when nothing moves.

The group of a molecule that will not hold still

A point group is a group of rotations of space, and it presupposes a structure for them to act on. Ethane has no one structure — its methyl groups turn billions of times a second — and the group that describes its spectrum has thirty-six elements where the point group has twelve, out of two thousand eight hundred and eighty conceivable.

symmetry · Point group

Named alongside it

The objects these essays reach for when they reach for this one.

Point groupAxialConjugacy classCoordination numberDegeneracyEquatorialEquivalent atomsGroup orderMinimisationParity (g and u)RepulsionSchoenflies symbols

All concepts