concentration in the blood. Insulin
was theÜrst protein whose amino-
acid sequence was fully determined
(in 1955). Underproduction of insulin
results in the accumulation of large
amounts of glucose in the blood and
its subsequent excretion in the urine.
This condition, known as diabetes
mellitus, can be treated successfully
by insulin injections.
intensive variableA quantity in a
macroscopic system that has a well
deÜned value at every point inside
the system and that remains (nearly)
constant when the size of the system
is increased. Examples of intensive
variables are the pressure, tempera-
ture, density, speciÜc heat capacity at
constant volume, and viscosity. An
intensive variable results when any
*extensive variable is divided by an
arbitrary extensive variable such as
the volume. A macroscopic system
can be described by one extensive
variable and a set of intensive vari-
ables.
intercalation cell A type of sec-
ondary cell in which layered elec-
trodes, usually made of metal oxides
or graphite, store positive ions be-
tween the crystal layers of an elec-
trode. In one type, lithium ions form
an intercalation compound with a
graphite electrode when the cell is
charged. During discharge, the ions
move through an electrolyte to the
other electrode, made of manganese
oxide, where they are more tightly
bound. When the cell is being
charged, the ions move back to their
positions in the graphite. This back-
wards and forwards motion of the
ions has led to the name rocking-
chair cell for this type of system. Such
cells have the advantage that only
minor physical changes occur to the
electrodes during the charging and
discharging processes and the elec-
trolyte is not decomposed but simplyserves as a conductor of ions. Conse-
quently, such cells can be recharged
many more times than, say, a lead-
acid accumulator, which eventually
suffers from degeneration of the elec-
trodes. Lithium cells, based on this
principle, have been used in portable
electronic equipment, such as cam-
corders. They have also been consid-
ered for use in electric vehicles.intercalation compound A type
of compound in which atoms, ions,
or molecules are trapped between
layers in a crystal lattice. There is no
formal chemical bonding between
the host crystal and the trapped mol-
ecules (see also clathrate). Such com-
pounds are formed by *lamellar
solids and are often nonstoichiomet-
ric; examples are graphitic oxide
(graphite–oxygen) and the mineral
*muscovite.interhalogenA chemical com-
pound formed between two *halo-
gens. Interhalogens are highly
reactive and volatile, made by direct
combination of the elements. They
include compounds with two atoms
(C1F, IBr, etc.), four atoms (C1F 3 , IF 3 ,
etc.), six atoms (BrF 5 , IF 5 , etc.) and IF 7
with eight atoms.
intermediate bondSee chemical
bond.
intermediate couplingSee j-j
coupling.intermetallic compoundA com-
pound consisting of two or more
metallic elements present in deÜnite
proportions in an alloy.
intermolecular forces Weak
forces occurring between molecules.
See van der waals’ force; hydrogen
bond.internal conversion A process in
which an excited atomic nucleus de-
cays to the *ground state and the en-
ergy released is transferred byintensive variable 286i