Isomerism is a very broad topic however they are molecules with the same molecular formula but different structured differently.

Isomers can be split into 2 groups-

1. Structural Isomers
2. Stereoisomers

Structural Isomers come in three main types- Chain – Same molecular formula but have a different chain length. E.g. Pentane and 2-methylbutane both have molecular formula of C5H12. Positional – Same molecular formula but different position of a functional group. E.g. 1-Chloropentane and 2-Chloropentane have the same molecular formula of C5H11Cl. Functional group – Same molecular formula but a different functional group. E.g. Pent-1-ene and Cyclopentane (one is an alkane and one is an alkene) both have the molecular formula of C5H10.

I have made a video that will explain more as part of my A Level Chemistry Revision videos YouTube channel. Structural Isomers

Stereoisomerism comes in the following forms-

Optical (enantiomers) – Same molecular formula but different optical properties. They exist as a pair of enantiomers which are mirror images of each other and are non-superimposable. They rotate plane polarised light by the same amount but in opposite directions. They have a chiral centre with 4 different groups coming from it. Your hand is chiral, a left hand glove will not fit a right hand as it is non-superimposable.

Geometric (E-Z isomerism) This is the same molecular formula but a different arrangement of atoms in space. E-Z or Cis-Trans. My video explains this in more detail. Stereoisomerism

Conformational Isomerism – Same molecular formula but structured differently due to rotations around a single covalent bond.