Transition metal

In chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings:

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• It commonly refers to any element in the d-block of the periodic table, including zinc and scandium. This corresponds exactly to periodic table groups 3 to 12 inclusive.
• More strictly, it can refer to those elements which form at least one ion with a partially filled d shell of electrons. This is exactly the d-block with zinc and scandium excluded.

Both definitions have their uses and supporters. The first has the attraction of apparent simplicity and is the traditional usage. However, many interesting properties of the transition elements as a group are the result of their ability to contribute valence electrons from s orbitals before d orbitals, a property which all members of the d-block except zinc and scandium share, so the more restricted definition is in many contexts the more useful. The d orbitals are contributed after the s orbitals because once the d orbital begins to fill its electrons move closer to the nucleus, leaving the s electrons as the outermost.

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Transition metal: The 40 transition metals ►