following rules provide explanations for how electrons are arranged within
principle energy levels.
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.)
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
- 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:
- 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.