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Transition metals are mainly classified as the d-block elements (groups 3–12), though group 12 elements are sometimes excluded and lanthanides/actinides are often treated separately as inner transition metals.
In chemistry, “transition metals” are defined primarily by their position on the periodic table: the elements in the d-block, corresponding to groups 3 to 12 (with some conventions allowing group 12 elements to be excluded). The IUPAC 2011 Principles of Chemical Nomenclature describe a transition metal as any element in groups 3–12, which matches the d-block definition used by many scientists. In practice, the f-block lanthanides and actinides are often treated separately as “inner transition metals,” though they are sometimes included under the broader idea of transition metals. The classification is also linked to electronic structure. Transition metals have atoms with partially filled d subshells (or can form cations with incomplete d subshells), and their d electrons (typically between 0 and 10 in the d-block) strongly influence their chemical behavior. As electrons are added across a transition series, they fill the same (n−1)d subshell, producing characteristic trends distinct from s- and p-block elements. Some periodic-table placements (notably involving group 3) can vary across published tables, and group 12 elements (Zn, Cd, Hg) are sometimes excluded because their d subshell is filled (d10) in their common oxidation states, though they may still show transition-metal-like bonding under certain criteria.
Transition metals are mainly classified as the d-block elements (groups 3–12), though group 12 elements are sometimes excluded and lanthanides/actinides are often treated separately as inner transition metals.
The defining feature is electronic: transition metals have partially filled d subshells, and filling of the (n−1)d orbitals across a series drives their distinctive chemical properties.
Periodic-table layouts can vary for heavier elements (especially group 3), and the placement of lanthanum/actinium versus lutetium/lawrencium is a known point of discussion.
An element in the periodic table’s d-block (groups 3–12), typically characterized by partially filled d subshells or cations with incomplete d subshells.
The set of elements whose chemistry is dominated by the filling of the (n−1)d subshell, corresponding to groups 3 through 12 in the standard periodic table.
A term commonly used for the lanthanides and actinides (f-block elements), which are often treated separately from the d-block transition metals.
A rule predicting the order in which subshells fill as atomic number increases, often used to rationalize typical electron configurations in transition metals.
A convention in which Zn, Cd, and Hg are sometimes not classified as transition metals because they have a filled d subshell (d10) in their common oxidation states.
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