valence The maximum number of univalent atoms
(originally hydrogen or chlorine atoms) that can com-
bine with an atom of the element under consideration,
or with a fragment, or for which an atom of this ele-
ment can be substituted.

valence bond theory Covalent bonds are formed
when atomic orbitals on different atoms overlap and
the electrons are shared.

valence electrons Outermost electrons of atoms;
often involved in bonding.

valence isomer A constitutional isomer interrelated
with another by PERICYCLIC REACTIONs. For example,
Dewar benzene, prismane, and benzvalene are valence
isomers of benzene.
See alsoTAUTOMERISM.

valence shell Valence electrons are the electrons
located in the outermost, highest-energy orbits or
SHELLof an atom. The shell is more of a field density
and indicates the region where the electrons are
located. The valence electrons determine the chemical
properties of an element, since it is these valence elec-
trons that are gained or lost during a chemical reaction.

valence-shell electron-pair repulsion theory The

repulsion between pairs of valence electrons is used to

predict the shape or geometry of a molecule.

valence tautomerization The term describes simple

reversible and generally rapid ISOMERIZATIONs or

DEGENERATE REARRANGEMENTs involving the forma-

tion and rupture of single and/or double BONDs, with-

out MIGRATIONof atoms or GROUPs; for example,

See alsoFLUXIONAL; TAUTOMERISM.

van der Waals equation Extends the IDEAL GAS LAW

to real gases by including two empirically determined

parameters, which are different for different gases.

van der Waals forces The attractive or repulsive

forces between MOLECULAR ENTITIES (or between

groups within the same molecular entity) other than

those due to BONDformation or to the electrostatic

interaction of ions or of ionic GROUPs with one another

or with neutral molecules. The term includes: DIPOLE-

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