The Periodic Table of the Elements is a graphic device that delineates all the known elements according to atomic number, electron distribution, and chemical properties. Of these, approximately two-thirds are metals. Some are described as alkali metals and some as alkaline earth metals. There are the Lanthanide metals and some others not assigned particular classification. The metals included in Groups III to XII of the Table are classified as transition metals.
Of particular importance in determining what is and what is not a transition metal is its electronic structure. Electrons bound to an atom are not free to travel anywhere in space, but, as explained by the laws of quantum mechanics, are restricted to specific shells, subshells, and orbitals. Each of these has a definite geometrical shape defining the volume in which the electron may travel.
An electron shell is defined by the principle quantum number and is sometimes called the principle energy level. It is usually identified by number: the 1st shell, the 2nd shell, and so on. Frequently the only unfilled shell is the outermost shell, but in the case of transition elements, the outermost two shells-both-are not totally filled. This is due to an energy overlap of the shells in larger atoms, beginning with scandium and the 3rd and 4th shell.
Shells can contain, from the 1st to the 4th shell respectively, 2, 8, 18, and 32 electrons. Due to another quantum number, these are subdivided in subshells. These are assigned letters, including s, p, d, f, and g respectively. The s subshell can hold only 2 electrons, p can hold 6, d can hold 10, f can hold 14, and g can hold 18. Within a shell, these are filled in order. However, not all subshells within a shell are filled before a new one is begun. In particular, the transition metal scandium has two electrons in the 4th shell while there is only one electron in the 3d subshell.
The official definition (See References 1) for a transition metal is, therefore, “An element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.” To illustrate: nickel has the electronic structure 1s²2s²2p⁶3s²3p⁶4s²3d⁸. Because it is not closed, the 3d subshell is listed last. If it were filled, it would contain 10 electrons.
Transition Metal Compounds
One interesting hallmark of transition metal compounds due to their unique electron structure is that they usually exhibit intense colors. Iron can produce red compounds, nickel green, uranium yellow, and cobalt blue compounds.
- International Union of Pure and Applied Chemistry: Red Book – transition element
- University of California – Davis: Transition Metal Chemistry