complementarityThe concept
that a single model may not be ade-
quate to explain all the observations
made of atomic or subatomic sys-
tems in different experiments. For
example, electron diffraction is best
explained by assuming that the elec-
tron is a wave (see de broglie wave-
length), whereas the photoelectric
effect is described by assuming that
it is a particle. The idea of two differ-
ent but complementary concepts to
treat quantum phenomena wasÜrst
put forward by the Danish physicist
Niels Bohr (1855–1962) in 1927.
complex A compound in which
molecules or ions form coordinate
bonds to a metal atom or ion (see il-
lustration). Often complexes occur as
complex ions, such as [Cu(H 2 O) 6 ]2+or
Fe[(CN) 6 ]3–. A complex may also be a
neutral molecule (e.g. PtCl 2 (NH 3 ) 2 ).
The formation of such coordination
complexes is typical behaviour of
transition metals. The complexes
formed are often coloured and have
unpaired electrons (i.e. are paramag-
netic). See also ligand; chelate.
complex ionSee complex.
complexometric analysis A type
of volumetric analysis in which the
reaction involves the formation of an
inorganic *complex.
componentA distinct chemical
species in a mixture. If there are no
reactions taking place, the number of
components is the number of sepa-
rate chemical species. A mixture of
water and ethanol, for instance, has
two components (but is a single
phase). A mixture of ice and water
has two phases but one component
(H 2 O). If an equilibrium reaction oc-
curs, the number of components is
taken to be the number of chemical
species minus the number of reac-
tions. Thus, in
H 2 + I 2 ˆ2HIthere are two components. See also
phase rule.compoundA substance formed by
the combination of elements inÜxed
proportions. The formation of a com-
pound involves a chemical reaction;
i.e. there is a change in the conÜgura-
tion of the valence electrons of the
atoms. Compounds, unlike mixtures,
cannot be separated by physical
means. See also molecule.comproportionation A reaction
in which an element in a higher oxi-
dation state reacts with the same el-
ement in a lower oxidation state to
give the element in an intermediate
oxidation state. For example
Ag2+(aq) + Ag(s) →2Ag+(aq)
It is the reverse of *disproportiona-
tion.Compton, Arthur Holly
(1892–1962) US physicist, who be-
came professor of physics at the Uni-
versity of Chicago in 1923. He is best
known for his discovery (1923) of the
*Compton effect, for which he
shared the 1927 Nobel Prize for
physics with C. T. R. Wilson.Compton effect The reduction in
the energy of high-energy (X-ray or
gamma-ray) photons when they are
scattered by free electrons, which
thereby gain energy. The phenome-
non,Ürst observed in 1923 by A. H.
*Compton, occurs when the photon
collides with an electron; some of the
photon’s energy is transferred to the
electron and consequently the pho-
ton loses energy h(ν 1 – ν 2 ), where h is
the *Planck constant and ν 1 and ν 2
are the frequencies before and after
collision. As ν 1 >ν 2 , the wavelength
of the radiation increases after the
collision. This type of inelastic scat-
tering is known as Compton scatter-
ingand is similar to the *Raman137 Compton effect
c