In quantum mechanics, spin-exchange is an interaction process between two particles mediated by an exchange interaction. It preserves total angular momentum of the system but may allow other aspects of the system to change. When two spin-polarized atoms in their ground state experience a spin-exchange collision, the total spin of the atoms is preserved yet the orientation of the individual spins may change. For example, if atoms A and B are oppositely polarized, a spin-exchange collision reverses the spins: