in chemistry, some molecules have more than one isomer. this means that molecules can have different forms, even though all the forms are made of the same atoms. there are two kinds of isomers. constitutional isomers have the same atoms, but they are joined differently. stereoisomers have the same atoms, they are joined the same way, but the atoms are arranged differently in space. an important part of stereochemistry is the study of chiral molecules. these molecules look almost identical, except that one molecule is the mirror image of the other.