Question

Why are electrons paired in orbits?

Answer 1

Quantum chemistry (quantum physics) tells us that each orbital can hold two electrons. The Pauli Exclusion Principle tells us that no two electrons in the same atom can have the same four quantum numbers; therefore, these two electrons may have the same n,l, and m_l so they may NOT have the same m_s. Thus we write one with m_s = +1/2 and the other electrons with m_s = -1/2.