In organic chemistry, an isomer (/ˈaɪsəmər/; from Greek ἰσομερής, isomerès; isos = "equal", méros = "part") is a molecule with the same molecular 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 two main forms of isomerism (/ˈaɪsəmərɪzm/ or /aɪˈsɒmərɪzm/): structural isomerism (or constitutional isomerism) and stereoisomerism (or spatial isomerism).