Transition element

A transition element is a chemical element whose atomic electron configuration of the ground (lowest energy) state has an incompletely filled d sub-shell. The symbol "d" stands for an atomic orbital with angular momentum quantum number ℓ = 2. The electron configuration of transition element atoms contains the orbital occupancy (nd)k, 1 &le; k &le; 9,  where n is the principal  quantum number of the d-orbital. The incomplete electronic d subshell gives rise to some characteristic magnetic properties (paramagnetism and ferromagnetism) and crystals and solutions of transition metal complexes that are brightly colored. In Table II, taken from NIST, it is shown that neutral transition element atoms also have one or two electrons in an s orbital with principal quantum number one higher than that of the partially filled d sub-shell.

Table I shows the part of the Periodic Table that contains the first three series of transition elements with principal quantum number n = 3, 4, and 5, respectively. The elements in the fourth transition series (period 7 of the periodic table), are formally transition elements. They are man-made [except for Actinium (Z = 87)] and short-lived, not much is known about their compounds  and accordingly they are not shown in Table I and II, and not discussed in this article.

Exceptions to the definition
Although the atoms copper (Cu), silver (Ag), and gold (Au) have a filled d sub-shell&mdash;as Table II shows they have the configuration (nd)10(n+1)s1, with n = 3, 4, and 5, respectively&mdash;after ionization (loss of two or more electrons) their cations have an incomplete d sub-shell. Since these cations appear in many complexes, copper, silver, and gold are usually seen as transition elements.

In the past, the group 12 elements zinc (Zn), cadmium (Cd), and mercury (Hg), that are included in the "d-block" of the periodic table, have been considered as transition elements, but they are nowadays rarely considered as such, because their compounds lack some of the properties that are characteristic for transition elements.

Because scandium (Sc), yttrium (Y), and lanthanum (La) actually do not form compounds analogous to those of the other transition elements and because their chemistry is quite homologous to that of the lanthanoids (previously known as lanthanides), they are often excluded from the group of transition elements. A strict application of the definition would describe also lutetium (Lu) as a transition element as it has a singly occupied 5d orbital in its ground state. According to IUPAC lutetium is a lanthanoid. It appears most commonly as a positive ion without d-electrons in the valence shell and without the characteristic properties of a transition element.

Properties
The most striking similarities shared by the transition elements is that they are all metals&mdash;which is why they are often called transition metals&mdash;and that most of them are hard, strong, and lustrous. They have high melting and boiling points, and, being metals, are good conductors of heat and electricity. Many of the elements are technologically important: iron, nickel, cobalt, titanium, platinum, and others are used in heterogeneous catalysis. They accelerate chemical reactions in which organic molecules are isomerized, built up from simple molecules, oxidized, hydrogenated, or polymerized. Much of the current research on the chemistry of transition element complexes is instigated by their industrial importance as catalysts.

Several of the metals and their compounds are ferromagnetic. Ferromagnetism is a collective effect that appears in the solid state and is due to the aligning of the unpaired electron spins of the metal ions. At low temperature several compounds, notably the oxides MnO, FeO, CoO, and NiO, make a phase transition to   antiferromagnetic form.

The transition elements form many useful alloys, among themselves and with other metallic elements. Most of the transition elements can be dissolved in water and other polar solvents by the action of acids and form complexes in solution, although  the "noble" metals platinum, silver, and gold are difficult to dissolve by non-oxidizing acids. For obvious reasons the elements copper, silver, and gold are referred to as coinage metals. Note that copper belongs to the class of coinage metals, but is not a noble metal.

Several transition elements are important to the chemistry of living systems, the most familiar examples being iron, cobalt, copper, and molybdenum. Iron is by far the most widespread and important transition metal that has a function in living systems; proteins containing iron participate in two main processes, oxygen transport and electron transfer (i.e., oxidation–reduction) reactions. There are also a number of substances that act to store and transport iron itself.

Ionic bonding
The outer s-electrons of the transition metals are easily lost to the bonding partners (the ligands) of the metal. Also one or more d-electrons of the metal are usually lost to its ligands. In other words, most transition element compounds show ionic chemical bonds. Common ligands are: oxide (O2&minus;), halides (F&minus;, Cl&minus;, Br&minus;,  I&minus;), hydrates (H2O, OH&minus;), cyanide (CN&minus;), and sulfate (SO42&minus;). Ligands are either negative ions, such as Cl&minus; or neutral molecules with one or more free electron pairs, such as water.

The formal charge of the ionically bound element is known as its oxidation number, or oxidation state. Table III shows the most common oxidation states of the first transition series. Note in this table that the elements exhibit variable oxidation states. The chemistry of the transition series is mainly that of the ions in one of their several  oxidation states, and not that  of the elemental form itself.

For example, the transition element chromium (Cr) in the ionic water complex chromium hexahydrate,  Cr(H2O)63+, is trivalent and is denoted by the oxidation state Cr(III). (This is because water has formal oxidation number zero.) This very commonly occurring triply charged cation has electronic structure [Ar](3d)3; it appears for example also in the crystal KCr(SO4)2&sdot;(H2O)12. The chromium in Cr(CN)64&minus; is divalent, denoted by Cr(II); it has  electronic structure [Ar](3d)4. Chromate [CrO4]2&minus; contains Cr(VI), which is isoelectronic with argon. An example of monovalent Cr(I) is in the bright-green compound K3[Cr(CN)5NO]&sdot;H2O, which contains K+, Cr+, NO+, and CN&minus;.

This widely applied classification of  transition elements by their oxidation states is not supported by quantum mechanical calculations. Although many theoretical discussions of transition metal complexes assume (often implicitly) ionic bonds, quantum mechanical calculations show that most of the bonds have a good deal of covalent character. This means that transition metal atomic orbitals are mixed (combined linearly) with orbitals on the ligands, thus forming molecular orbitals. Calculations bear out that transfer of more than one full electron to the ligands occurs rarely, let alone six electrons as in Cr(VI). However, in qualitative and semi-quantitative studies, the assumption of ionic bonds with  a transition metal cation provides much insight and yields a systematization of the properties of the transition metal complexes. The covalent character of the bonds is then accounted for by adjustment of the values of the semi-empirical parameters that enter such studies.

Abundance of the transition elements
Iron is the most common transition element in the Earth's solid crust. It takes fourth place among the elements  and is the  second metal in the crust after aluminum. The elements titanium, manganese, zirconium, vanadium, and chromium are abundant and appear in concentrations larger than 100 grams per ton. Some of the most important and useful transition elements are rare, for instance, tungsten, platinum, gold, and silver. Obviously they are among the most expensive of the transition metals.

In antiquity the elements iron (ferrum), copper (cuprum), silver (argentum), and gold (aurum) were already widely known. The other regular transition elements were recognized as elements from the early 18th century onward when analytic chemistry  techniques were refined. Rhenium (Z = 75) was the latest transition metal discovered in nature; it was detected in 1925 in platinum ores and in the niobium mineral columbite. The element is extremely rare, no concentrated ores have been found thus far.

The element technetium (Z = 43) is man-made, it was first made in 1937 by bombarding molybdenum with deuterons, and has so far not been found in nature. All isotopes of technetium are radioactive, although the half-lifes of three of the common isotopes are long (Tc-97: 2.6&times;106, Tc-98: 4.2&times;106, and Tc-99: 210&thinsp;000 years). The Tc isotopes can be isolated in considerable quantities from the fission products of nuclear reactors.

Theory

 * See Crystal field theory

In general, the relative magnitudes of d orbital splittings for a given ion with different ligands is determined by the ligands. This ordering of ligands according to their ability to split the energies of the d orbitals is called the spectrochemical series. From weakest to strongest the splittings are: I&minus; < Br&minus; < Cl&minus; <  OH&minus; < F&minus; < H2O < pyridine < NH3 < ethylenediamine < CN&minus;.