I.

Introduction

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.

II.

Properties

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. 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.

III.

Uses

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.

IV.

Classification

In the periodic table, the transition elements are arranged into groups 3 (or IIIb) through 12 (or IIb) (see Periodic Law).