1. Electron Configuration
When in their most stable configuration, called the ground-state, the arrangement of electron orbitals can be organized according to three rules, or principles:
The aufbau principle states that each electron occupies the lowest energy level available (electrons generally “fill in” from the nucleus outward).
The Pauli exclusion principal states that the two electrons found in an orbital have opposite “spins” (not really a spin, but close enough for our class), represented by “up” and “down” arrows: ↑↓
Hund’s rule states that single electrons of identical spin must occupy each equal-energy orbital before additional electrons of opposite spin can occupy the same orbital. In a p-energy level, for example, the first 3 electrons occupy ↑ ↑ ↑ , not ↑↓ ↑

2. Aufbau Principle
All orbitals related to an energy sublevel are of equal energy (px, py, and pz are all of equal energy).
Different energy sublevels within a principle energy level have different energies (the 2p sublevel has higher energy than the 2s sublevel).
In order of increasing energy within a principle energy level, the sublevel sequence is s, p, d, and f.
There can be overlap of energy amounts between orbitals of different primary energy levels. The 4s energy sublevel has less energy than the 3d energy sublevel, even though n = 4 is farther from the nucleus than n = 3.

3. Aufbau Principle
The sequence of orbital filling, from lowest energy to highest, is:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p...
Worried about how you’re going to learn this? Try this to make it easier: 1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f

4. Electron Configuration
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p...
Oxygen has 8 electrons. According to the aufbau principle:
- 2 are found in the 1s orbital,
- 2 are found in the 2s orbital, and
- 4 are found in the 2p orbitals (px, py, and pz)
According to the Pauli exclusion principle, the two electrons in 1s have opposite “spins” (as do the two electrons in 2s).
According to Hund’s rule, the 2px, 2py, and 2pz orbitals each get one of the four remaining electrons, and all three of these electrons “spin” in the same direction. The fourth electron completes one of the orbitals (it doesn’t matter which one), and “spins” in the opposite direction.

5. Orbital Diagrams and Electron Configuration
An orbital diagram summarizes the location and spin of the electrons surrounding the nucleus of an atom.
Oxygen would be written ↑↓ ↑↓ ↑↓ ↑ ↑ .
1s s p
Another way to write is by electron configuration notation, which lists the principle energy levels and sublevels involved, and the number of electrons found in each sublevel.
Oxygen would be written 1s22s22p4.

6. Electron Configuration
Silicon has 14 electrons.
According to the aufbau principle, 2 are found in the 1s orbital, 2 are found in the 2s orbital, 6 are found in the 2p orbitals (px, py, and pz), 2 are found in the 3s orbital, and 2 are found in the 3p orbitals.
According to the Pauli exclusion principle, the two electrons in 1s have opposite “spins” (as do the two in 2s, the six in 2p, and the 2 in 3s).
According to Hund’s rule, the 3px and 3py orbitals (or 3px and 3pz - it doesn’t matter, as long as it is two different orbitals) each get one of the two final electrons, and both of these electrons “spin” in the same direction.
Orbital diagram of Si: ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑ ↑
1s s p s p
Electron configuration notation of Si: 1s22s22p63s23p2.

7. Valence Electrons and Electron Dot Structures
Those electrons found in the outermost principle energy level, called valence electrons, are the ones that give an element its chemical properties.
Oxygen, written as 1s22s22p4, has 6 valence electrons (2 in 2s and 4 in 2p).
Silicon, written as 1s22s22p63s23p2, has 4 valence electrons (2 in 3s and 2 in 3p).
Electron dot structures help visualize these valence electrons:
O Si