|Table of contents|
|What are Bond Parameters?|
|Resonance in Chemical Bonding|
|London Dispersion Forces|
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Covalent bonds can be characterized on the basis of several bond parameters such as bond length, bond angle, bond order, and bond energy (also known as bond enthalpy). These bond parameters offer insight into the stability of a chemical compound and the strength of the chemical bonds holding its atoms together.
(i) Size of the atoms: Greater the size of the atoms, greater is the bond length and less is the bond dissociation enthalpy, i.e., less is the bond strength.
(ii) Multiplicity of bonds: For the bond between the same two atoms, greater is the multiplicity of the bond, greater is the bond dissociation enthalpy. This is firstly because atoms come closer and secondly, the number of bonds to be broken is more. For example, bond dissociation enthalpies of H2, O2 and N2 are in the order: H–H < O = O < N ≡ N
(iii) Number of lone pairs of electrons present: Greater the number of lone pairs of electrons present on the bonded atoms greater is the repulsion between the atoms and hence less is the bond dissociation enthalpy. For example, for a few single bonds, we have
The equilibrium distance between the centres of the nuclei of the two bonded atoms is called its bond length.
The angle between the lines representing the directions of the bonds, i.e., the orbitals containing the bonding electrons is called the bond angle.
The formal charge on an atom in a molecule or ion is defined as the difference between the number of valence electrons of that atom in the free state and the number of electrons assigned to that atom in the Lewis structure, assuming that in each shared pair of electrons, the atom has one electron of its own and the lone pair on it belongs to it completely.
Thus, it can be calculated as follows:
Calculate formal charge on each O-atom of O3 molecule.
Sol. Lewis structure of O3 is:
The atoms have been numbered as 1, 2 and 3.
Formal charge on end O-atom numbered 1
Formal charge on central O-atom numbered 2Formal charge on end O-atom numbered 3Hence, we represent O3 along with formal charges as:
Write the formal charges on atoms in (i) carbonate ion (ii) nitrite ion.
Sol. (i) Lewis structure of CO32– ion is
Formal charge on C atom
Formal charge on double bonded O atomFormal charge on single bonded O atom(ii) Lewis structure of NO2– ion is Formal charge on N atomFormal charge on double bonded O atomFormal charge on single bonded O atomSignificance of formal charge:
The main advantage of the calculation of formal charges is that it helps to select the most stable structure, i.e., the one with least energy out of the different possible Lewis structures. The most stable is the one which has the smallest formal charges on the atoms.
Resonance in Chemical Bonding
There are molecules and ions for which drawing a single Lewis structure is not possible. For example, we can write two structures of O3.
In (A) the oxygen-oxygen bond on the left is a double bond and the oxygen-oxygen bond on the right is a single bond. In B the situation is just the opposite. The experiment shows, however, that the two bonds are identical.
Therefore neither structure A nor B can be correct. One of the bonding pairs in ozone is spread over the region of all three atoms rather than localized on a particular oxygen-oxygen bond. This delocalized bonding is a type of chemical bonding in which bonding pair of electrons are spread over a number of atoms rather than localized between two.
Structures (A) and (B) are called resonating or canonical structures and (C) is the resonance hybrid. This phenomenon is called resonance, a situation in which more than one canonical structure can be written for a species. The chemical activity of an atom is determined by the number of electrons in its valence shell. With the help of the concept of chemical bonding, one can define the structure of a compound and is used in many industries for manufacturing products in which the true structure cannot be written at all.
Some other examples:
- CO32– ion
- Carbon-oxygen bond lengths in carboxylate ion are equal due to resonance.
The difference in the energies of the canonical forms and resonance hybrid is called resonance stabilization energy.
London Dispersion Forces
Another form of chemical bonding is caused by London dispersion forces. These forces are weak in magnitude.
London Dispersion Forces
These forces occur due to a temporary charge imbalance arising in an atom. This imbalance in the charge of the atom can induce dipoles on neighbouring atoms. For example, the temporary positive charge on one area of an atom can attract the neighbouring negative charge.