Moco SeeMOLYBDENUM COFACTOR.
model A synthetic COORDINATIONentity that closely
approaches the properties of a METALion in a PROTEIN
and yields useful information concerning biological
structure and function. Given the fact that the term is
also loosely used to describe various types of molecular
structures (constructed, for example, in the computer),
the term BIOMIMETICis more appropriate.
moderator A substance such as hydrogen, deu-
terium, oxygen, or paraffin used in a nuclear reactor to
slow down the NEUTRON.
moiety In physical organic chemistry, moiety is gen-
erally used to signify part of a molecule, e.g., in an
ester R^1 COOR^2 , the alcohol moiety is R^2 O. The term
should not be used for a small fragment of a molecule.
molality Concentration term expressed as number of
moles of solute per kilogram of solvent.
molarity The number of moles of solute dissolved in
1 liter of solution.
molar solubility Number of moles of a solute that
dissolve to produce a liter of saturated solution.
mole (mol) An amount of substance that contains as
many items such as ions, molecules, etc., as the number
of atoms in exactly 12 grams of carbon (C). The num-
ber of molecules contained is equal to 6.022 × 1023
(602,200,000,000,000,000,000,000), known as Avo-
gadro’s number. Therefore a mole is anything that has
Avogadro’s number of items in it.
molecular entity Any constitutionally or isotopically
distinct atom, MOLECULE, ion, ION PAIR, RADICAL, RAD-
ICAL ION, COMPLEX, conformer, etc., identifiable as a
separately distinguishable entity.
The term molecular entityis used in this glossary
as a general term for singular entities, irrespective of
their nature, while CHEMICAL SPECIESstands for sets or
ensembles of molecular entities. Note that the name of
a compound may refer to the respective molecular
entity or to the chemical species, e.g., methane may
mean a single molecule of CH 4 (molecular entity) or a
molar amount—specified or not (chemical species)—
participating in a reaction.
The degree of precision necessary to describe a
molecular entity depends on the context. For example,
“hydrogen molecule” is an adequate definition of a cer-
tain molecular entity for some purposes, whereas for
others it is necessary to distinguish the electronic state
and/or vibrational state and/or nuclear spin, etc., of the
hydrogen molecule.molecular equation Any equation for a chemical
reaction where all formulas are written as if all sub-
stances exist as molecules.molecular formula The formula of a compound in
which the subscripts give the number of each element
in the formula.molecular geometry The arrangement of atoms
around a central atom of a molecule or polyatomic ion;
the general shape of a molecule determined by the rela-
tive positions of the atomic nuclei.molecular graphics The visualization and manipula-
tion of three-dimensional representations of molecules
on a graphical display device.molecularity The number of reactant MOLECULAR
ENTITIESthat are involved in the “microscopic chemical
event” constituting an ELEMENTARY REACTION. (For
reactions in solution, this number is always taken to
exclude molecular entities that form part of the
MEDIUMand that are involved solely by virtue of their
solvation of solutes.) A reaction with a molecularity of
one is called “unimolecular”; one with a molecularity
of two is “bimolecular”; and a molecularity of three is
“termolecular.”molecularity 181