The following rules provide explanations for how electrons are arranged within principle energy levels.

Aufbau Principle - Electrons enter obitals of lowest energy first.

• The orbitals of a subshell are all equal. For example, the p subshell has 3 orbitals. All have the same energy.
• Within an energy level, the s subshell is the lowest energy sublevel. The f subshell is the highest energy sublevel. The p & d sublevels are in the middle but d has more energy than p.. (Energy in sublevels: s < p < d < f)
• Energy levels sometimes overlap but since electrons enter obitals of lowest energy first, electrons will fill a 4s orbital before a 3d. This is why electron configurations fill in the order they do, even if the energy levels are not in order. (For example, the folowing snippet in order is: 6s2 4f14 5d10 6p6 . A 6s orbital fills before a 4f and 5d because of the Aufbau Principle.)

Pauli Exclusion Principle - An atomic orbital can describe at most two electrons.

• Atomic orbitals are represented using a box.
• If an orbital contains only one electrons, it would be written like this:
• In order for two electrons to fill the same orbital, they must have opposite spins. One spins clockwise and one spins counterclockwise. An orbital with two electrons would look like this:

Hund's Rule - When electrons occupy orbitals of the same energy, electrons will enter empty orbitals first.

• A p subshell with three orbitals would all have one electron with parallel spins before any orbital will gain a second electron.
  The example would represent a p3 subshell.