Aufbau Principle
- Electrons occupy the lowest energy orbital first
Hund's Rule
- Degenerate orbitals (orbitals with the same energy level) are occupied first by single electrons with up spin before they are occupied by paired electrons
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Energy: Aufbau Principle & Hund's Rule
So next, let's factor in the property of energy. To do so, we'll consider how to position electrons in multiple orbitals with different energies on a vertical scale.
- Write that according to the Aufbau [Off-bau] principle, electrons occupy the lowest energy orbital first
- They will fill orbitals in a way that achieves the most stable, lowest-energy configuration.
- Consider that they will also fill orbitals with the same energy level (degenerate orbitals) in the lowest energy manner, as well.
- Write that according to Hund's rule, degenerate orbitals (orbitals with the same energy level) are occupied first by single electrons with "up" spin before they are occupied by paired electrons.
- Having the same spin ensures that the electrons will not pair until all the orbitals have been singly occupied.
- Now, let's apply the Aufbau principle and Hund's rule in the orbital diagram for oxygen, which has the configuration [1s^2 2s^2 2p^4].
- Indicate the direction of increasing energy.
- Draw the 1s orbital.
- Draw the 2s orbital above the 1s orbital.
- Draw the three 2p orbitals as three lines side-by-side, above the 2s orbital.
- Now, add the electrons for each orbital.
- Show the two 1s electrons as a pair in the 1s orbital, first.
- Show the two 2s electrons as a pair occupying the 2s orbital, second.
- In accordance with Hund's rule, add one electron with "up" spin to each of the three p orbitals.
- Then add the last electron with "down" spin to pair with one of the other three.