The term transition metal (sometimes also called a transition element) has two possible meanings:
- In the past it referred to any element in the d-block of the periodic table, which includes groups 3 to 12 on the periodic table. All elements in the d-block are metals (In actuality, the f-block is also included in the form of the lanthanide and actinide series).
- The modern, IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.
Jensen has reviewed the historical usage of the terms transition element (or metal) and d-block. The word transition was first used to describe the elements now known as the d-block by the English chemist Charles Bury in 1921, who referred to a transition series of elements during the change of an inner layer of electrons (for example n=3 in the 4th row of the periodic table) from a stable group of 8 to one of 18, or from 18 to 32.
In the d-block the atoms of the elements have between 1 and 10 d electrons.
With a few minor exceptions, the electronic structure of transition metal atoms can be written as [ ]ns2(n-1)dm, where the inner d orbital has more energy than the valence-shell s orbital. In divalent and trivalent ions of the transition metals, the situation is reversed such that the s electrons have higher energy. Consequently, an ion such as Fe2+ has no s electrons: it has the electronic configuration [Ar]3d6 as compared with the configuration of the atom, [Ar]4s23d6.
The elements of groups 4–11 are now generally recognized as transition metals, as are Sc and Y in Group 3. For the elements La-Lu and Ac-Lr and also for Group 12, different sets of definitions are used by different authors.
Zinc, cadmium, and mercury are not classified as transition metals as they have the electronic configuration [ ]d10s2, with no incomplete d shell. In the oxidation state +2 the ions have the electronic configuration [ ] d10. While these elements can exist in the +1 oxidation state, as in the diatomic ion Hg2+
2, there are no unpaired electrons because of the formation of a covalent bond between the two atoms of the dimer. The group 12 elements Zn, Cd and Hg may be classed as post-transition metals. However, it is often convenient to include these elements in a discussion of the transition elements. For example, when discussing the crystal field stabilization energy of first-row transition elements, it is convenient to include the non-transition elements calcium and zinc, as both Ca2+ and Zn2+ have a value of zero against which the value for other transition metal ions may be compared. Another example occurs in the Irving-Williams series of stability constants of complexes.
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