Isomers vs Resonance | Resonance Structures vs Isomers | Constitutional Isomers, Stereoisomers, Enantiomers, Diastereomers
A molecule or ion having the same molecular formula can exist in different ways depending on the bonding orders, charge distribution differences, the way they arrange themselves in the space etc.
Isomers
Isomers are different compounds with the same molecular formula. There are various types of isomers. Isomers can be mainly divided into two groups as constitutional isomers and stereoisomers. Constitutional isomers are isomers where the connectivity of atoms differs in molecules. Butane is the simplest alkane to show constitutional isomerism. Butane has two constitutional isomers, butane itself and isobutene.
CH3CH2CH2CH3 
Butane Isobutane/ 2-methylpropane
In stereoisomers atoms are connected in the same sequence, unlike constitutional isomers. Stereoisomers differ only in the arrangement of their atoms in space. Stereoisomers can be of two types, enantiomers and diastereomers. Diastereomers are stereoisomers whose molecules are not mirrored images of each other. The cis trans isomers of 1, 2-dichloroethene are diastereomers. Enantiomers are stereoisomers whose molecules are nonsuperposable mirror images of each other. Enantiomers occur only with chiral molecules. A chiral molecule is defined as one that is not identical with its mirror image. Therefore, the chiral molecule and its mirror image are enantiomers of each other. For example, 2-butanol molecule is chiral, and it and its mirror images are enantiomers.
Resonance
When writing Lewis structures, we only show valence electrons. By having the atoms share or transfer electrons, we try to give each atom the noble gas electronic configuration. However, at this attempt, we may impose an artificial location on the electrons. As a result, more than one equivalent Lewis structures can be written for many molecules and ions. The structures written by changing the position of the electrons are known as resonance structures. These are structures which only exist in theory. The resonance structure states two facts about the resonance structures.
- None of the resonance structures will be the correct representation of the actual molecule; none will completely resemble the chemical and physical properties of the actual molecule.
- The actual molecule or the ion will be best represented by a hybrid of all the resonance structures.
The resonance structures are shown with the arrow ↔. Following are the resonance structures of carbonate ion (CO32-).
X-ray studies have shown that the actual molecule is in between these resonances. According to the studies, all the carbon-oxygen bonds are in equal length in carbonate ion. However, according to the above structures we can see one is a double bond, and two are single bonds. Therefore, if these resonance structures occur separately, ideally there should be different bond lengths in the ion. The same bond lengths indicate that none of these structures actually present in nature, rather a hybrid of this exists.
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What is the difference between Isomers and Resonance? • In isomers, atomic arrangement or spatial arrangement of the molecule may differ. But in resonance structures, these factors do not change. Rather, they only have a change in position of an electron. • Isomers are naturally present, but resonance structures do not exist in reality. They are hypothetical structures, which are restricted only to theory.
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