isostere

Some parts in a machine are interchangeable because they are the same size and shape, even if made of different metal. An isostere is a chemical part like that: an atom or group that can replace another because the two are similar in size, shape, and electron arrangement, so a molecule barely notices the swap physically.

Historically the term came from physics — atoms or groups with the same number of atoms and the same count of valence electrons were called isosteric, like the classic pairs N2 and CO, or –O–, –NH–, –CH2– and –S– as divalent bridges. In medicinal chemistry, isosteres are groups judged similar enough in steric and electronic terms that substituting one for another is a reasonable design move when reshaping a molecule's properties.

Isosterism is a guiding heuristic about physical similarity, not a guarantee of biological equivalence. Two formally isosteric groups can differ sharply in pKa, hydrogen-bonding ability, lipophilicity, or metabolic fate, so an isosteric swap that looks conservative on paper may change activity or ADMET in practice. When the replacement specifically preserves biological activity while improving properties, it is called a bioisostere.