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Transition metal
In chemistry, the term transition metal (or transition element) has three possible meanings:
  • The IUPAC definition defines a transition metal as "an element whose atom has a partially filled  d sub-shell, or which can give rise to cations with an incomplete d sub-shell".
  • Many scientists describe a "transition metal" as any element in the d-block of the periodic table, which includes groups 3 to 12 on the periodic table. In actual practice, the f-block lanthanide and actinide series are also considered transition metals and are called "inner transition metals".
  • Cotton and Wilkinson follow the IUPAC definition (see above) which includes scandium and yttrium on the grounds that they at least have a partially filled d subshell in the metallic state. However even this does not (so far) seem to confer catalytic properties on them which is so characteristic of the transition metals in general. Lanthanum and actinium are included with the lanthanides and actinides respectively.

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