CLASS :- 11
Topic :-Resonance
Resonance
The concept first appeared in 1899 in Johannes Thiele's as a "Partial Valence Hypothesis"
The mechanism of resonance was introduced into quantum mechanics by Werner Heisenberg in 1926
Lewis dot structures can be drawn to visualize the electrons and bonds within a certain molecule. However, for some molecules not all the bonding possibilities and properties cannot be represented by a single Lewis structure; these molecules have several contributing or “resonance” structures.
In chemistry terms, resonance explain the fact that electrons are delocalized, or flow freely through with in the molecule, which allows multiple structures to be possible for a given molecule.
Each contributing resonance structure can be visualized by drawing a Lewis structure; however, it is important to note that each of these structures cannot actually be observed in nature. That is, the molecule does not actually go back and forth between these configurations; rather, the true structure is an approximate intermediate between each of the structures. This intermediate has an overall lower energy than each of the possible configurations and is referred to as a resonance hybrid. It is important to note that the difference between each structure lies in the location of the electrons and not in the arrangement of the atoms.
resonance structures of ozone molecules
The ozone (O3) molecule consists of a central oxygen atom which is singly bonded to one oxygen atom and doubly bonded to another. There is no net charge on this molecule, but the Lewis structures of this molecule show a +1 charge on the central oxygen and a -1 charge on the singly bonded oxygen
In Ozone molecules two type Bond length 148 pm and 121 pm but actual Bond length are 128 pm
it means the bond between oxygen atom intermediate between single and double bond
resonance structures of Carbonate Ion
Our first attempt at drawing the lewis dot structure of the polyatomic carbonate ion results in the structure shown below. Carbonate has 24 electrons, and carry the -2 charge
resonance structures of benzene
In benzene the two cyclohexatriene Kekulé structures, first proposed by Kekulé
resonance energy
The difference in energy between the actual molecule and the canonical form of the lowest energy is called the Resonance Energy. The resonance energy of a molecule is expressed in kcal/mole or kJ/mole.
It is the difference between the actual bond energy of the molecules and and that of the most stable of the resonance structures
resonance energy = actual bond enthalpy - energy of the most stable of Canonical structures of molecules
For example the resonance energy of carbon dioxide 138.2 Kilo joule per mole means actual molecules of carbon dioxide almost 138.2 kj per Mol more stable than the most stable structures among the resonance structures
Resonance energy = E3 -Eo
characteristic of resonance
1 Only the resonance hybrid has the real exercise and other resonance structures only imaginary propose to explain the properties of the molecules
2 the bond length in a resonating structures become equal
3 that is hybrid has lowest energy level and greater stability than other resonance structures
4 greater the resonance structures and resonance energy e the molecules have maximum stability
5 this concept is only theoretical concept