An isomer (; from Greek ἰσομερής, isomerès; isos = "equal", méros = "part") is a molecule with the same chemical formula as another molecule, but with a different chemical structure. That is, isomers contain the same number of atoms of each element, but have different arrangements of their atoms. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like positional isomers, cis-trans isomers and enantiomers, etc. (see chart below). There are two main forms of isomerism ( or ): structural isomerism and stereoisomerism (spatial isomerism).