Transition metal

A transition metal is an element whose atom has a partially filled d subshell or can form cations with a partially filled d subshell. This electronic structure underlies its characteristic variable oxidation states and coordination chemistry.

d-block: The periodic-table region in which the differentiating electron enters a d subshell. Most transition metals occupy this region, though d-block membership alone does not define the category.
Crystal field theory: A model that explains transition-metal d-orbital energy changes through electrostatic interactions with surrounding ligands. It connects ligand arrangement to the splitting of d orbitals in metal complexes.
Iron: A chemical element with symbol Fe and atomic number 26. Iron illustrates variable oxidation states in both biological systems and industrial materials.
Zinc: A chemical element with symbol Zn and atomic number 30, whose common ion has a filled d subshell. Zinc sits in the d block but is excluded under definitions requiring a partially filled d subshell.
Electron configuration: The arrangement of electrons among an atom's shells, subshells, and orbitals. A partially filled d subshell is the defining electronic feature in the stated criterion.
Ligand field theory: A theory of metal–ligand bonding that combines crystal-field ideas with molecular orbital descriptions. It accounts for covalent contributions that a purely electrostatic account omits.
Copper: A chemical element with symbol Cu and atomic number 29. Copper's redox chemistry supports electron transfer in organisms and electrical conductivity in devices.
Cadmium: A chemical element with symbol Cd and atomic number 48, whose common ion has a filled d subshell. Cadmium is another d-block element that usually fails the partially filled d-subshell criterion.
Aufbau principle: A model for predicting electron configurations by filling orbitals in order of increasing energy. It gives a starting account of d-electron placement, with notable exceptions among transition metals.
d–d transition: An electronic excitation between split d orbitals in a transition-metal ion or complex. Absorption of visible light during these excitations produces many characteristic complex colors.

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