Isomers
Isomers are compounds with the same molecular formulae but different structural formulae.
They have the same number of each type of atom but may have different physical and chemical properties.
This diagram shows butane and methylpropane. (Methylpropane used to be called iso-butane, but the correct name is methylpropane).
Both have a molecular formula of \({C_4}{H_{10}}\). The structural formulae are different though, so they are isomers of each other.
Isomers are molecules with the same formula but different chemical structures.