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Isomer: Molecule has same molecular formula but different structural arrangement is called isomer.
Geometric isomerism (also known as E-Z isomerism or cis-trans isomerism): same molecular formula but different arrangement in the space. These isomers happen where you have restricted rotation in a molecule (double bond in the molecule). The functional groups are same side in the molecule is cis, the functional groups are opposite side is called trans isomer.
Constitutional isomer: (or structural) isomers differ in the connectivity they contain different functional groups and / or bonding patterns is called constitutional isomer.
Resonance: it is a process of delocalization electrons with in the molecule.
Chiral: Absence of a plane of symmetry or a center of symmetry is called chiral molecule, a non-superimposable on its mirror image is called chiral. A carbon atom is attached by the four different groups is called chiral carbon.
Enantiomers: Two stereoisomers that are mirror images of each other and they are non-supposable