Isosteres are molecules or ions with the same number of atoms and the same number of valence electrons.
Some examples of isoteres include:
The isostere concept was formulated by Irving Langmuir in 1919,[1] and later modified by Grimm. Friedrich Erlenmeyer extended the concept to biological systems in 1932.[2] Classical isosteres are defined as being atoms, ions and molecules that had identical outer shells of electrons, This definition has now been broadened to include groups that produce compounds that can sometimes have similar biological activities. Some evidence for the validity of this notion was the observation that some pairs, such as benzene and thiophene, thiophene and furan, and even benzene and pyridine, exhibited similarities in many physical and chemical properties.
A biologically-active compound containing an isostere is called a bioisostere. This is frequently used in drug design: the bioisostere will still be recognized and accepted by the body, but its functions there will be altered as compared to the parent molecule.