Transition Elements, series of chemical elements that share similar electron orbital structures and hence similar chemical properties. The transition elements are commonly defined as the 30 elements with atomic numbers 21 to 30, 39 to 48, and 71 to 80.
The transition elements exhibit multiple valences or oxidation states typically ranging from +1 to +8 in compounds. In organometallic compounds, consisting of metals bonded to organic species, transition metals sometimes take on negative oxidation states. The transition elements have such typical metallic properties as malleability, ductility, high conductivity of heat and electricity, and metallic luster. They tend to act as reducing agents (donors of electrons), but are less active in this regard than the alkali metals and alkaline earth metals, which have valences of +1 and +2, respectively. There are exceptions, as in the case of mercury (Hg), which is a liquid Transition elements in general have high densities and melting points and exhibit magnetic properties. They form both ionic and covalent bonds with anions (negatively charged ions), and such compounds are in general brightly colored.. They have high electrical conductivity because of delocalization of the s electrons similar to what occurs in the alkali and alkaline-earth metals. Another characteristic of the transition metals is the great variety of oxidation states shown in its compounds.
Several transition elements and their compounds are important catalysts (see Catalysis) in a variety of industrial processes, especially in the manufacture of petroleum and plastic products, where organic molecules are hydrogenated, oxidized, or polymerized (see Chemical Reaction; Hydrogenation; Polymer). Compounds of titanium, aluminum, or chromium are used in the polymerization of ethylene to form polyethylene. Catalysts containing iron are used in preparing ammonia from hydrogen and nitrogen. Molecules containing transition elements are important to the biochemical processes in many living systems, the most familiar example of which is the iron-containing heme complex of hemoglobin, which is responsible for oxygen transport in the blood of all vertebrates and some invertebrates. Most transition metals are colored and make some of their ionic compounds colored. This is because they absorb some of the frequencies of white light. This is attributed to electronic transitions in the d subshell, separating them into different levels of energy. When light is absorbed, an electron is raised from a lower state to a higher state, giving the rise to color. The stored energy is then dissipated through heat.
The transition metals also have complex ionic structures because of the availability of d orbitals for participating in chemical bonding.