Transition element
(Under construction)
A transition element is a chemical element element whose atomic electron configuration of the ground (lowest energy) state has an incompletely filled d sub-shell. Here d stands for an atomic orbital with angular momentum quantum number ℓ = 2. The electron configuration of free transition element atoms contains (nd)k, with 1 ≤ k ≤ 9, and where n is a principal quantum number, n = 3, 4, 5. The incomplete electronic d subshell gives rise to some characteristic magnetic properties and brightly colored solutions of transition metal complexes.
Although the atoms copper (Cu), silver (Ag), and gold (Au) have in their lowest energy state a filled d sub-shell (nd)10, after ionization (loss of two or more electrons) they obtain an incomplete d sub-shell; hence, they are usually seen as transition elements. As seen in the table of electron configurations below, Cu, Ag, and Au have the outer configuration: (nd)10(n+1)s1, with n = 3, 4, and 5 for copper, silver, and gold, respectively. 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 often been considered as transition elements, but they are nowadays rarely considered as such, because their compounds lack some of the characteristic properties.
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 lanthanide), they are often excluded from the group of transition elements. Also a strict application of the definition would describe lutetium (Lu) as a transition element as it has a singly occupied 5d orbital in its ground state, but according to IUPAC[1] it 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.
| Group | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
|---|---|---|---|---|---|---|---|---|---|
| 1st series | Sc (21) | Ti (22) | V (23) | Cr (24) | Mn (25) | Fe (26) | Co (27) | Ni (28) | Cu (29) |
| 2nd series | Y (39) | Zr (40) | Nb (41) | Mo (42) | Tc (43) | Ru (44) | Rh (45) | Pd (46) | Ag (47) |
| 3rd series | La (57) | Hf (72) | Ta (73) | W (74) | Re (75) | Os (76) | Ir (77) | Pt (78) | Au (79) |
The first three series of the transition elements are shown in the two tables. The elements in the fourth 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 discussed in this article.
Properties
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The most striking similarities shared by the transition elements is that they are all metals—which is why they are often called transition metals—and that most of them are hard, strong, and shiny. 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, palladium, platinum, and others are used in heterogeneous catalysis. Much of the current research on the chemistry of transition element complexes is instigated by their industrial importance as catalysts.
The transition elements form many useful alloys, among themselves and with other metallic elements. Most of these elements can be dissolved in water and other polar solvents and form complexes in solution, although the "noble" metals platinum, silver, and gold are difficult to dissolve. For obvious reasons the elements copper, silver, and gold are often referred to as coinage metals[2]. Note that copper belongs to the class of coinage metals, but is not a noble metal.
The elements exhibit variable valences and form stable compounds in several formal oxidation states (ions of certain formal charge). 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 instance the element chromium (Cr) in the ionic water complex Cr(H2O)63+ is trivalent and is denoted by the oxidation state Cr(III). (This is because water has formal oxidation number zero.) The Cr(III) cation has electronic structure [Ar](3d)2. The chromium in Cr(CN)64− is divalent, denoted by Cr(II), and has electronic structure [Ar](3d)4. Chromate [CrO4]2− contains Cr(VI), which is isoelectronic with argon. Note, parenthetically, that this classification of transition elements in different oxidation states, although still widely applied, is not supported by quantum mechanical calculations. It is rare that complete transfer of even one full electron occurs, let alone 6 as in Cr(VI).
Reference
- ↑ IUPAC Provisional Recommendations for the Nomenclature of Inorganic Chemistry (online draft of an updated version of the "Red Book" IR 3-6), 2004. Retrieved on 17/9/2009.
- ↑ B. H. Lipshutz and Y. Yamamoto. Introduction, Special issue of Chemical Reviews on Coinage Metals in Organic Synthesis, 2008, vol. 108, pp. 2793–2795 DOI