The transition elements - sometimes referred to as the transition metals - are a group of metallic elements in the centre of the periodic table. They appear in the d-block, to the right of Groups 1 and 2 (the alkali and alkaline earth metals) and to the left of Groups 3-6 (the metalloids and non-metals).

The d-block of elements looks like this:

21  22  23  24  25  26  27  28  29  30
Sc  Ti  V   Cr  Mn  Fe  Co  Ni  Cu  Zn
39  40  41  42  43  44  45  46  47  48
Y   Zr  Nb  Mo  Tc  Ru  Rh  Pd  Ag  Cd
    72  73  74  75  76  77  78  79  80
    Hf  Ta  W   Re  Os  Ir  Pt  Au  Hg
   104 105 106 107 108 109 110 111 112
    Rf  Db  Sg  Bh  Hs  Mt Unn Uuu Uub

The transition elements can be defined as "elements which contain an incomplete d orbital in at least one compound." The d orbital contains the electrons from 8-17 in each electron shell. d electrons are found in the elements from scandium onwards (proton number 21). Some transition elements are of course household names: iron, nickel, copper, gold and silver for example. Others are much more obscure, e.g. molybdenum and seaborgium.

All the d-block elements except scandium (Sc) and zinc (Zn) fit the above definition, and thus the transition elements are those of the d-block minus these two exceptions. Scandium is an exception because the ions in its compounds lose their only d electron, thus they have no d orbital at all. Zinc is the other exception because its atoms have a complete d orbital (ten electrons) and its ions lose only the two outer s electrons, so it retains a complete d orbital. Scandium behaves more like aluminum than the other d-block elements, while zinc has more in common with the alkaline earth metals (Group 2).

The transition elements have a number of other common characteristics. Aside from all being metallic, their atomic radii all have comparable (and relatively small) values. They are therefore dense and tough (with the exception of mercury, a liquid), and the similarity in radii allows them to form alloys, such as brass, bronze and steel.

Many of their compounds are coloured, both when solid and in solution. This is because many transition metal ions have energy levels capable of absorbing certain wavelengths of visible light in order to promote electrons. When light strikes such an ion, it is reflected back as a mixture of the wavelengths which have not been absorbed, which we perceive as a colour. Most ions of non-transition metals, by comparison, cannot absorb any frequencies of visible light (they absorb ultraviolet radiation instead) so white light reflects back with all its wavelengths intact, and thus remains white.

One of the more familiar coloured transition compounds is copper(II) sulphate, CuSO4. When solid or in aqueous solution it is bright blue, particularly striking when concentrated. In other solvents it has a different colour - green in chloride ions (Cl-1) and brown in benzene-1,2-diol (C6H4(OH)2), for example.

All transition elements have variable oxidation numbers, meaning they lose a different number of electrons in different compounds. Copper, for example, can have an oxidation number of +1 or +2, while chromium can be +2, +3 or +6. Contrast with the metals of Groups 1 and 2, which have non-varying oxidation numbers of +1 and +2 respectively.

This characteristic is the reason that so many of the transition elements can act as catalysts, either in their elemental form or in compounds. Since they can readily change their oxidation number, they can change that of the reactants too. For example permanganate ions, MnO4-1, which contain the transition metal manganese, can be used to catalyse the decomposition of hydrogen peroxide, H2O2.

When in solution, transition metal ions can form complexes. These are compounds in which several molecules (often four, five or six) of a species known as a ligand each donate one or more lone pairs of electrons to the vacant d-orbitals of a transition metal ion, forming a dative covalent bond. The ligands coordinate with a particular structural geometry around the metal centre, hence the name "complex". Water itself acts as a ligand when transition metal ions are in aqueous solution, but it can be displaced by other ligands, such as ammonia:

[Cu(H2O)4(aq)]2+ + 4NH3 (aq) --> [Cu(NH3)4(aq)]2+ + 4H2O(l)

Complex ions form because they are more stable than the corresponding transition ion, owing to the more even spread of positive charge. This phenomenon and its consequences are described by what is called crystal field theory.

Reference: Michael Volkins (general editor), Nuffield Advanced Chemistry, 2000

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