Bond Parameters
There are different features or characteristics of bonds which can be termed as bond parameters. The covalent bond is characterized by following parameters:
1. Bond Length
2. Bond Angle
3.The Bond Enthalpy
4. Bond Order
What is Bond Length?
Bond length is the average distance between the centers of the nuclei of two bonded atoms in a molecule. Shorter the bond length, larger is the value of bond energy. It is expressed in Angstrom units (Å) or picometers (pm).
1Å = 10-10m and 1pm = 10 -12-1) is smaller than that of C=C bond (619 kJ mol-1).
What is Size of the atom?
The bond length increases with increase in the size of the atom. For example, C-C < Si-Si < Ge-Ge This is due to the increase in the distance of the electrons from the nucleus with the addition of a new shell. Therefore the average distance between the bonding nuclei (bond length) increases. Smaller the bond length, smaller will be the size of the atom as atoms were bounded more tightly.
What is Bond Angle?
The average angle between the orbitals of the central atom containing the bonding electron pairs in the molecule is known as bond angle between the atoms. The unit of bong angle is either degree or minute or second. This gives an idea about the distribution of orbital’s around the central atom in a molecule e. Therefore bond angle determines the shape of a molecule.
For example, the H-O-H bond angle in H2O is 104.5° and H-N-H bond angle in NH3 is 107°.
Rules for estimating stability of resonance structures
1. The greater the number of covalent bonds, the greater the stability since more atoms will have complete octets
2. The structure with the least number of formal charges is more stable
3. The structure with the least separation of formal charge is more stable
4. A structure with a negative charge on the more electronegative atom will be more stable
5. Positive charges on the least electronegative atom (most electropositive) is more stable
6. Resonance forms that are equivalent have no difference in stability and contribute equally
What is Bond Enthalpy
Tge bond enthalpy is directly proportional to the strength of the bond between the molecules. The amount of energy which is require in order to break one mole of the bond of a particular type between two atoms in a gaseous state is referred to as the Bond Enthalpies.
For polyatomic molecules, there is different bond enthalpy even if the two bonds of the same type .
For example : Two O-H bonds of water molecule got different bond enthalpy. The polyatomic molecules have average bond enthalpy due to differences in bond enthalpy.
What is Bond Order
Lewis describe the covalent bonds, the bond order is the number of bonds that forms in between the two atoms in a molecule. The Isoelectronic molecules or ions have the same bond order.
For example: the two isoelectronicc molecules, F2 and O22- are isoelectronic molecules have the same bond order of 1. In the case of greater order of bond, there is an increase in bond enthalpy and decrease in the length bond.
H – H Bond order = 1
O = O Bond order = 2
N ≡ N Bond order = 3
C ≡ O Bond order = 3