Ligand

a gemstone’s colour is the size of a gap. adjust it by hand.

drag ligands onto the six sites around a metal ion, one bond at a time, and watch the solution find a new colour

A transition-metal ion in solution is never bare -- something is always bonded to it in all six positions around it, starting with plain water. Whatever those six ligands are, their electric field splits the metal’s d-orbitals into two energy levels, and the size of that gap decides which colour of light the complex absorbs. A stronger-field ligand -- cyanide is a strong one, chloride a weak one -- widens the gap, which shifts the absorbed wavelength shorter, which shifts what reaches your eye: you see whatever colour was NOT absorbed. Swap even one of the six ligands and the average field the ion feels moves, which is most of the actual difference between a ruby and an emerald.

Concepts: chemistry, symmetry.

An interactive you play with one thumb, on Wundr.