Appendix I. Glossary
Prepared by I. D. Brown
idbrown@mcmaster.ca
This appendix defines the terms and conventional symbols used in bond valence theory. Words
written in italics are defined in this appendix.
Actual coordination number
The coordination number observed for an atom in a given compound
Anion
The atom with the larger electronegativity forming a bond. Anions are uncharged except
in the ionic model where the anion carries a negative charge equal to its negative valence.
Anion bonding strength, SB
The expected value of the valence of a bond formed by an anion. It can be estimated by
dividing the anion's (negative) valence by its expected coordination number. This is the
same as Lewis base strength.
Anion valence, VB
The valence of an atom when acting as an anion. VB is a negative number.
Antibonding region
A region on the side of an atom opposite to the bond. Electrons in this region tend to
weaken the bond
Atom type
An atom of a particular element with a particular valence (oxidation state)
Atomic valence, V
The number of electrons in the valence shell that an atom uses in bonding. The units of
atomic valence are valence units, equivalent to one bonding electron. Cation valences
are taken as positive, anion valences as negative
Bipartite graph
The graph of a bond network in which the atoms are of two kinds (e.g., cation or anion)
with no bonds occurring between atoms of the same kind.
Bond
The chemical link between two neighboring atoms. In the ionic model, two atoms are
bonded if and only if they are linked by electrostatic flux.
Bond network
A topological description of the way in which atoms in a system are linked by bonds.
Bond strain index
Root mean square deviation between the observed and ideal bond valence averaged over
all the bonds in the formula unit.
Bond valence, S
The number of valence electrons an atom uses to form a given bond. In the ionic model
it is equal to the electrostatic flux linking two ions. The units of bond valence are valence
units (vu), equivalent to one bonding electron pair. Bond valences are considered to be
directed from the cation to the anion.
Bonding electrons
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The electrons in the valence shell of an atom that are used for forming bonds.
Bonding region
The region between two atoms where the valence shells overlap. Electrons in this region
form the bond
Bonding strength
An estimate of the valence of the bonds formed by an ion. See anion- and
cation-bonding strength, Lewis acid- and base-strength.
Cation
The atom with the smaller electronegativity forming a bond . Cations are uncharged
except in the ionic model where the cation carries a positive charge equal to its positive
valence.
Cation bonding strength, SA
The expected value of the valence of a bond formed by a cation. It can be estimated by
dividing the cation's (positive) valence by its expected coordination number. This is the
same as the Lewis acid strength.
Cation valence, VA
The valence of an atom when acting as a cation. VA is a positive number.
Chemical bond
A localized interaction between two neighboring atoms. See bond
Complex
A group of two or more atoms linked together by strong (primary or covalent) bonds.
Complexes may be ions carrying a formal charge (residual valence) or neutral molecules.
Coordination number, N
The number of bonds formed by, or the number of atoms surrounding, a given atom. See
actual coordination number
Coordination sphere
The environment of an atom including all its ligands.
Covalent bond
A bond may be referred to as a covalent bond if it I has a large bond valence. A primary
bond is usually regarded as a covalent bond.
Electron pair bond
A bond of valence 1.0 vu involving exactly one pair of electrons
Electronegativity, SE
The property of an element that gives a measure of the electric potential at the surface of
the atomic core. The scale used in the bond valence model sets the electronegativity
equal to the maximum (cation) bonding strength of the atom. This is equal to the
number of electrons in the valence shell divided by the average coordination number
adopted by the atom in this valence state when bonded to oxygen ligands.
Electrostatic flux
The electrostatic flux linking two atoms in the ionic model is the bond valence.
External bond
A bond formed between an atom in a complex and an atom that is not part of the complex.
In general the valence of an external bond is smaller than that of a bond internal to the
complex.
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Global instability index, G (also GII)
The root mean square deviation of the bond valence sums from the atomic valences
averaged over all atoms in the formula unit.
Heteroionic bond
A bond formed between a cation and an anion.
Homoionic bond
A bond formed between two cations or between two anions
Ideal bond valences
Bond valences predicted for a valence compound using the network equations
Ionic model
A model in which every atom is assigned as either a cation or an anion, each carrying a
charge equal to its valence. A bond exists in this model between two atoms if they are
linked by electrostatic flux. This model can be applied to any valence compound without
regard to whether the bonding is ionic or covalent.
Lewis acid strength, SA
The cation bonding strength, specifically when this is the bonding strength for the
external bonds formed by an atom of a complex. It is equal to the residual valence of an
atom acting as a Lewis acid divided by the expected number of external bonds.
Lewis base strength, SB
The anion bonding strength, specifically when this is the bonding strength for the
external bonds formed by an atom of a complex. It is equal the residual valence of an
atom acting as a Lewis base divided by the expected number of external bonds.
Ligand
An atom bonded to the atom under consideration.
Lone pair
A pair of valence shell electrons not involved in bonding.
Network equations
The Kirchhoff equations used to solve the capacitive electric circuit equivalent to the
bond network of a valence compound.
NO
The average coordination number when oxygen is the ligand.
Overbonded
Refers to an atom whose bond valence sum exceeds its atomic valence
Overlap region
The region in which the valence shells of two atoms overlap to form a bond.
Primary bond
A bond with a larger than average valence, formed by an atom with one or more
stereoactive lone pairs of electrons.
Residual valence
The valence available to an atom in a complex for forming external bonds after the
valences of the internal bonds have been satisfied.
SA
Lewis acid strength, see cation bonding strength
SB
Lewis base strength, see anion bonding strength
SE
Electronegativity
Secondary bond
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A bond with a smaller than average valence, formed by an atom with one or more
stereoactive lone pairs of electrons.
Surface instability index
The root mean squared deviation of the bond valence sum from the atomic valence
averaged over the atoms forming the surface of a solid.
Underbonded
Refers to an atom whose bond valence sum is less than its atomic valence
Valence of an atom, see Atomic valence
Valence of a bond, see Bond valence
Valence compound
A compound having a bond network with a bipartite graph. All the bonds have an anion
at one end and a cation at the other.
Valence shell
The outer electron shell of an atom containing the electrons involved in bonding. It
contains a number of bonding electrons equal to the valence of the atom, and possibly
non-bonding lone pairs.
Valence unit (vu)
1 valence unit of atomic valence corresponds to one electron, 1 vu of bond valence
corresponds to one pair of electrons since each atom contributes one electron to the bond.
Valence vector
A vector with the magnitude of the bond valence directed from the cation to the anion,
VA
Cation valence (positive)
VB
Anion valence (negative)
VE
Lone pair
VR
Residual valence
vu
Valence unit.
.
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