A tool which is used predominately in organic chemistry to representcertain types of molecular structures is called as resonance.
It is a key component of valence bond theory. When no singleconventional model can account for all the observed properties of themolecule, the concept of resonance raised and explains all thepossible electronic arrangement in molecule. We cannot use the Lewisdot diagrams to represent the true electronic structure of a moleculebut the resonance structures are employed to approximate the trueelectronic structure.
Thereare many molecules which cannot show all of their physical andchemical properties by using single structure, so more than onestructure have to be proposed.
Theresonance structures or canonical forms or contributing structureshave different arrangement of electrons but have identical positionsfor atoms.
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Rememberonly imaginary and the actual structure of the molecule is consideredas the hybrid of all the resonance structures and called as resonancehybrid whose energy is always less than the energy of any of thecontributing resonance structure, therefore becomes more stablecompare to other structures.
Allthe resonance structures are formed because of delocalization ofelectrons which can represent by using curved arrows. There is nochange in the positions of atoms. The resonance structures must bevalid Lewis structures obeying octet rule and should possess samenumber of electrons as in molecule. There must be no change in numberof lone pair and bond pair in molecule as well as their position inmolecule.
Theresonance is a stabilizing phenomenon as the resonance hybrid of amolecule has lower energy than any of the contributing form.
Asthe number of contributing structures increases, the stability ofmolecule increases. The resonance structures with full octets ofatoms are more stable compare to the ones with open octets.
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Let’s discuss the SO3Molecular Shape and resonance concept in molecule. According to VSEPRtheory, the Shape of SO3 is a trigonal planar withD3h point group. Here in Molecular Geometry SO3, thecentral atom that is sulphur bonded with three oxygen atoms. In SO3molecular geometry, it is a planar covalent molecule with one sulphuratom bonded with three oxygen atoms through double bond and singlebonds. One sulphur-oxygen double bond shows bond angle is 120while the other two oxygen atoms bonded with dative bond. Themolecule of SO3 shows resonance with three resonatingstructures. The central sulphur atom has an oxidation state of+6 and a formal charge of +2 which expands the octet ofmolecule and results a non-polar molecule.
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