Constitutional isomers, also known as structural isomers, are one or more compounds that have the same molecular formula, but whose atoms are connected in different sequences. As a result, constitutional isomers have different functional groups; that is, sets of atoms in a molecule that contribute to the molecule certain characteristic chemical and physical properties and bonding patterns, or ways of connecting to other molecules.
The four types of constitutional isomers are chain isomers, which vary in the way their carbons are connected to one another; functional isomers, which have different functional groups; positional isomers, whose functional groups vary in positioning on the same carbon chain and tautomers, which readily interconvert through a process called tautomerization.
Stereoisomers, or geometrical isomers, are one or more compounds having the same molecular formula and sequence of attached carbons, but whose three dimensional structures are different. Thus, they have the same functional groups and bonding patterns. When the spatial arrangements differ with respect to the reference plane, stereoisomers are often called cis-trans isomers, where "cis" and "trans" are used to denote atoms or groups that are either on the same side or opposite sides of a compound's double bond, respectively.
The kinds of stereoisomers are, first, enantiomers, also known as optical isomers, as they are stereoisomers that are mirror opposites of one another, but that are not superimposable upon one another (spatially identical in three dimensions, like right and left hands). These have the same chemical and physical properties, but are polarized differently. The second is the diastereoisomers, or diastereomers: These are all stereoisomers that are not enantiomers, so they are not superimposable and they are not mirror images. Unlike enantiomers, diastereoisomers have different chemical and physical properties.