Property characteristic of families of related subatomic
particles differing mainly in the values of their electric
charge. The families are known as isospin multiplets. The
components of atomic nuclei, the neutron and the proton, form an
isospin doublet since they differ only in electric charge and
subsidiary properties. They are considered different versions of
the same object, called a nucleon. The isospin of a nucleon has
a value of Learn more about isospin with a free trial on
Britannica.com. Thus, isospin was introduced as a concept well
before the development in the 1960s of the quark model which
provides our modern understanding. The nucleons, baryons of spin
, were grouped together because they both have nearly the same
mass and interact in nearly the same way. Thus, it was
convenient to treat them as being different states of the same
particle. Since a spin particle has two states, the two were
said to be of isospin . The proton and neutron were then
associated with different isospin projections Iz