i. Bond Length Bond length or bond distance is defined as the equilibrium internuclear separation distance of the bonded atoms in a molecule. It is measured by spectroscopic, X-ray diffraction and electron diffraction techniques.
In the case of covalent bond, the contribution from each atom is called covalent radius of that atom. In the periodic table the covalent radius increases down a group and they decrease for s and p-block elements from left to right across a period. The bond length decreases from single covalent bond to triple covalent bond.
ii. Bond Enthalpy Bond enthalpy, i.e., the strength of a chemical bond, is measured as the bond dissociation enthalpy. It is the enthalpy required to break a particular bond in one mole of gaseous molecule.
iii. Bond Order In the Lewis description of covalent bond, the Bond order is given by the number of bonds between the two atoms in a molecule.
The bond order, for example in H (with a single shared electron pair), in O (with two shared electron pairs) and in N (with three shared electron pairs) is 1, 2, 3 respectively. Similarly in CO (three shared electron pairs between C and O) the bond order is 3. For N the bond order is 3 and its ΔH is 946 kJ mol is one of the highest dissociation energy for a diatomic molecule. Isoelectronic molecules and ions have identical bond orders; for example, F and O have bond order 1. N O and NO have bond order 3.
A general correlation useful for understanding the stabilities of molecules is that with increase in bond order, bond enthalpy increases and bond length decreases.
iv. Bond Angle The angle between the orbitals containing bonding electron pairs around the central atom in a molecule/complex ion is called its bond angle. It is expressed in degree which can be experimentally determined by spectroscopic methods.
The H-O-H bond angle in water can be represented as follows:
v. Resonance Structures When a single structure is unable to explain all the properties of a compound, a number of other structures are suggested to that substance under certain roles. These various structures have no real existence and are called contributing, canonical or resonating structures. These are separated by the sign .