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.