An electron always goes into the available orbital with the lowest energy. REMEMBER: the
closer the atomic orbital is to the nucleus, the lower is its energy.
So a 1s orbital is lower in energy than a 2s orbital because the 1s
orbital is closer to the nucleus.
Pauli Exclusion Principle
No more than two electrons can occupy each atomic orbital
The two electrons must be of opposite spin
It is called the exclusion principle because it limits the number of electrons that can occupy any
From these first two rules, we can assign electrons to atomic orbitals for atoms that contain one, two, three, four, or five electrons.