transition metals

Meaning: Transition metals are the d-block elements (groups 3–12), where a d subshell is being filled. Strictly, an element counts only if its atom or a common ion has a partially filled d subshell, so Zn (Zn²⁺ is d¹⁰) is sometimes excluded; many courses treat the whole block together. Because ns and (n−1)d energies are close, these metals lose different numbers of electrons and show several oxidation states, such as Fe²⁺ and Fe³⁺. Their ions act as Lewis acids, binding ligands to form coordination complexes. Splitting of the d orbitals lets many complexes absorb visible light, so they are often colored, and unpaired d electrons make them paramagnetic.

On the MCAT: Expect to write an ion's configuration (Fe³⁺ is [Ar] 3d⁵), count unpaired electrons to predict paramagnetism, or find a metal's oxidation state from a complex's formula and ligand charges. Passages also use transition metals as catalysts (Pt, Pd, Ni) or as cofactors in metalloenzymes.

Common confusion: Students remove 3d electrons first because 3d fills after 4s. On ionization the 4s electrons leave first: Fe → Fe²⁺ gives [Ar] 3d⁶. Cr ([Ar] 4s¹3d⁵) and Cu ([Ar] 4s¹3d¹⁰) are the usual exceptions when filling.