bonds may be in the form of heat, electricity, or
mechanical means. Bond energy is the quantity of
energy that must be absorbed to break a particular
kind of chemical bond. It is equal to the quantity of
energy the bond releases when it forms. It can also be
defined as the amount of energy necessary to break one
mole of bonds of a given kind (in gas phase).
See alsoBOND-DISSOCIATION ENERGY.
bond energy (mean bond energy) The average value
of the gas-phase BOND-DISSOCIATION ENERGIES(usually
at a temperature of 298 K) for all BONDs of the same
type within the same CHEMICAL SPECIES. The mean
bond energy for methane, for example, is one-fourth
the enthalpy of reaction for
CH 4 (g)→C(g)+ 4H(g)
Tabulated bond energies are generally values of
bond energies averaged over a number of selected typi-
cal chemical species containing that type of bond.
bonding orbital A molecular orbital that is located
between two atomic nuclei, the orbitals of which over-
lap and reinforce each other. Electrons in a bonding
orbital tend to stabilize a molecule. The term also refers
to a molecular orbital, the energy of which is lower
than that of the atomic orbitals that are used in its con-
struction.
bonding pair A pair of electrons used to form a
covalent bond.
bond migration SeeMIGRATION.
bond number The number of electron-pair BONDs
between two nuclei in any given LEWIS FORMULA. For
example, in ethene, the bond number between the car-
bon atoms is two and between the carbon and hydro-
gen atoms is one.
bond order A theoretical index of the degree of
bonding between two atoms relative to that of a nor-
mal single bond, i.e., the bond provided by one local-
ized electron pair. In the VALENCE BOND THEORY, it is a
weighted average of the bond numbers between the
respective atoms in the CONTRIBUTING STRUCTUREs. In
MOLECULAR-ORBITAL THEORY, it is calculated from the
weights of the atomic orbitals in each of the occupied
molecular orbitals. For example, in valence-bond the-
ory (neglecting other than KEKULÉ STRUCTUREs), the
bond order between adjacent carbon atoms in benzene
is 1.5; in Hückel molecular-orbital theory, it is 1.67.
Other variants of molecular-orbital theory provide
other values for bond orders.bone imaging The construction of bone tissue images
from the radiation emitted by RADIONUCLIDEs that have
been absorbed by the bone. Radionuclides such as 18F,
85Sr, and 99mT c are introduced as complexes with spe-
cific LIGANDs (very often phosphonate ligands) and are
absorbed in the bones by metabolic activity.
See alsoIMAGING.borderline mechanism A mechanism intermediate
between two extremes, e.g., a nucleophilic substitution
intermediate between SN1 and SN2 or an intermediate
between electron transfer and SN2.Born-Haber cycle A series of thermochemical reac-
tions or cycles used for calculating the lattice energies
of ionic crystalline solids.boron hydrides Binary compounds of boron and
hydrogen.Bovet, Daniele (1907–1992) Swiss Physiologist
Daniele Bovet was born in Neuchâtel, Switzerland, on
March 23, 1907, to Pierre Bovet, professor of peda-
gogy at the University of Geneva, and Amy Babut. He
graduated from the University of Geneva in 1927 and
then worked on a doctorate in zoology and compara-
tive anatomy, which he received in 1929.
During the years 1929 until 1947, he worked at the
Pasteur Institute in Paris, starting as an assistant and
later as chief of the Laboratory of Therapeutic Chem-
istry. Here he discovered the first synthetic antihis-32 bond energy