A quantum number applied to hadrons (see elementary particles) to distinguish between members of a set of particles that differ in their electromagnetic properties but are otherwise apparently identical. For example, if electromagnetic interactions and weak interactions are ignored, the proton cannot be distinguished from the neutron in their strong interactions: isotopic spin was introduced to make a distinction between them. The use of the word ‘spin’ implies only an analogy to angular momentum, to which isotopic spin has a formal resemblance. The concept of isotopic spin is very useful both in nuclear physics and in the theory of elementary particles. For example, the existence of the neutral pi-meson was predicted on the basis of isotopic spin long before it was discovered experimentally.