Britannica Sources used in this answer

Why can transition metals form compounds with more than one oxidation state?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — transition metal Modern version of the periodic table of the elements (printable).

Transition metals are defined as elements that possess valence electrons in two shells rather than one, specifically occupying the middle portions of the periodic table in Groups 3 through 12.1

Their ability to form compounds with multiple oxidation states is primarily attributed to their partially filled d orbitals. These elements have electron configurations where the energy difference between the outer s electrons and the underlying d electrons is relatively small. Consequently, transition metals can lose varying numbers of these electrons to form stable ions or participate in covalent bonding.23

For elements in the first transition series, the highest possible oxidation state often corresponds to the total number of d and s electrons available. However, as the nuclear charge increases across the series, these electrons become more tightly bound, making higher oxidation states progressively more difficult to achieve. The specific oxidation state exhibited in a compound is further influenced by the chemical environment, as the surrounding ligands cause the d orbitals to separate into different energy levels, a phenomenon analyzed through ligand-field theory and molecular orbital theory.324