With Breit, this scientist names a two-component approximation that ignores terms larger than order of the speed of light squared. That approximation breaks down for elements 
With Breit, this scientist names a two-component approximation that ignores terms larger than order of the speed of light squared. That approximation breaks down for elements larger than the 2nd transition metal row and simplifies the four-component Dirac equation. Any two components of this scientist's namesake matrices have a commutation relation that is proportional to the third component of the matrices, which are the same commutation relationships as angular momentum. Those 2 by 2 matrices symbolized (*) sigma are multiplied by the reduced Planck's constant divided by two to give the spin operator. More famously, he described a rule that says two identical particles with anti-symmetric wavefunctions cannot occupy the same single particle state. That means the same quantum state cannot be occupied by two identical fermions. For 10 points, name this man who formulated a famous exclusion principle.
With Breit, this scientist names a two-component approximation that ignores terms larger than order of the speed of light squared.
Wolfgang Pauli