Transition Elements

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.

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