Isomers are molecules with the same molecular formula but a different arrangement of atoms.

You can also describe isomers as molecules with the same molecular formula but a different structural formula.

Structural isomerism involves different arrangements of atoms in the molecule.

Stereoisomerism involves the same arrangement of atoms but different spatial arrangements.

1. Chain isomerism.
2. Positional isomerism.
3. Functional group isomerism.

Positional isomers have the same carbon skeleton but differ in the position of functional groups.

For example, the positional isomers of chlorobutane are: 

Chain isomers have different arrangements of the carbon skeleton, including straight chains and branched chains.

For example, the chain isomers of C₄H₁₀ are:

Functional group isomers have the same atoms arranged into different functional groups.

For example, the functional group isomers of C₆H₁₂ are:

To indicate the position of the double bond in positional isomers of butene, use a number before the suffix to specify the carbon atom where the double bond starts, for example, but-1-ene or but-2-ene.

1. Geometric (cis-trans) isomerism.
2. Optical isomerism.

Cis-trans isomerism results from restricted rotation around carbon-carbon double bonds in alkenes when each carbon atom of the C=C double bond has the same two atoms or groups attached.