To a good approximation the proton and neutron have the same mass: they can be interpreted as two states of the same particle. These states have different values for an internal isospin coordinate. The mathematical properties of this coordinate are completely analogous to intrinsic spin angular momentum. The component of the operator, T ^ ^ 3 , for this coordinate has eigenvalues + ⁠ 1 / 2 ⁠ and − ⁠ 1 / 2 ⁠ ; it is related to the charge operator, Q ^ ^ : Q ^ ^ = e ( T ^ ^ 3 + 1 2 ) which has eigenvalues e for the proton and zero for the neutron. For a system of n nucleons, the charge operator depends upon the mass number A: Q ^ ^ = e ( T ^ ^ 3 + 1 2 A ) Isobars, nuclei with the same mass number like K and Ar, only differ in the value of the T ^ ^ 3 eigenvalue. For this reason isospin is also called "isobaric spin".