In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, bond energy (''BE''), also called the mean bond
enthalpy
Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
or average bond enthalpy is the measure of bond strength in a
chemical bond. IUPAC defines bond energy as the average value of the gas-phase
bond-dissociation energy (usually at a temperature of 298.15 K) for all bonds of the same type within the same chemical species.
The bond dissociation energy (enthalpy) is also referred to as
bond disruption energy, bond energy, bond strength, or binding
energy (abbreviation: ''BDE'', ''BE'', or ''D''). It is defined as the standard
enthalpy change of the following fission: R - ''X'' → R + ''X''. The ''BDE'',
denoted by Dº(R - ''X''), is usually derived by the thermochemical
equation,
:
The
enthalpy of formation Δ''H
f''º of a large number of atoms, free radicals, ions,
clusters and compounds is available from the websites of
NIST
The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...
,
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeedin ...
,
CODATA
The Committee on Data of the International Science Council (CODATA) was established in 1966 as the Committee on Data for Science and Technology, originally part of the International Council of Scientific Unions, now part of the International ...
, and
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
. Most authors prefer to use the
''BDE'' values at 298.15 K.
For example, the
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
–
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
bond energy in
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
''BE''
(C–H) is the enthalpy change (∆''H'') of breaking one molecule of methane into a carbon atom and four hydrogen
radicals, divided by four. The exact value for a certain pair of bonded elements varies somewhat depending on the specific molecule, so tabulated bond energies are generally averages from a number of selected typical chemical species containing that type of bond.
Bond energy (''BE'') is the average of all
bond-dissociation energies of a single type of bond in a given molecule. The bond-dissociation energies of several different bonds of the same type can vary even within a single molecule. For example, a
water molecule
Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "unive ...
is composed of two O–H bonds bonded as H–O–H. The bond energy for H
2O is the average of energy required to break each of the two O–H bonds in sequence:
:
Although the two bonds are the equivalent in the original symmetric molecule, the bond-dissociation energy of an oxygen–hydrogen bond varies slightly depending on whether or not there is another hydrogen atom bonded to the oxygen atom.
When the bond is broken, the bonding electron pair will split equally to the products. This process is called
homolytic bond cleavage (homolytic cleavage; homolysis) and results in the formation of radicals.
Predicting the bond strength by radius
Metallic radius,
ionic radius, and
covalent radius of each atom in a molecule can be used to estimate the bond strength. For example, the ''covalent'' radius of
boron is estimated at 83.0
pm, but the bond length of B–B in B
2Cl
4 is 175 pm, a significantly larger value. This would indicate that the bond between the two boron atoms is a rather ''weak'' single bond. In another example, the metallic radius of
rhenium is 137.5 pm, with a Re–Re bond length of 224 pm in the compound Re
2Cl
8. From this data, we can conclude that the bond is a very strong bond or a
quadruple bond. This method of determination is most useful for covalently bonded compounds.
Factors affecting ionic bond energy
The
electronegativity of the two atoms bonding together affects ionic bond energy.
Greater differences in electronegativity correspond to stronger ionic bonds.
See also
*
Binding energy
In physics and chemistry, binding energy is the smallest amount of energy required to remove a particle from a system of particles or to disassemble a system of particles into individual parts. In the former meaning the term is predominantly use ...
*
Ionization energy
*
Isodesmic reaction
*
Lattice energy
In chemistry, the lattice energy is the energy change upon formation of one mole of a crystalline ionic compound from its constituent ions, which are assumed to initially be in the gaseous state. It is a measure of the cohesive forces that bin ...
References
{{Authority control
Energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...
Binding energy