Walsh Diagram: This displays how the labels and energies of molecular orbitals of a molecule change with a change in the geometry of that molecule.

Wave-Particle Duality: The quantum mechanical result that links the classical ideas of a particle and a wave, saying that each has some of the properties of the other. It is formalised in the de Broglie relation.

Wavefunction: A mathematical function that describes absolutely the properties of the system. For most systems, especially macroscopic ones, the wavefunction will be too complex to be written out explicitly.

Wavenumber: A unit commonly used in spectroscopy, it is the reciprocal of the wavelength. If reported in units of cm-1 (as is common) then it can be interpreted as the number of complete wavelengths in 1cm. The frequency is equal to the wavenumber multiplied by the speed of light.

Weak Acid: An acid with pKa > 0.

Weak Field: This is when the effect of the field created by the ligand field is a small separation in energy of the d-orbitals.

Work: The movement of an object against an opposing force; the orderly movement of particles. Work is always equivalent to the raising of a weight somewhere in the surroundings.