For both SL and HL IB Chemistry, write an electron configuration by assigning the atom's electrons to orbitals in order of increasing energy, while applying the Aufbau principle, Hund's rule, and the Pauli exclusion principle.
The Three Rules
| Rule | How it controls electron arrangement |
|---|---|
| Aufbau principle | Electrons occupy the available orbitals of lowest energy first. The usual order begins . |
| Hund's rule | Electrons occupy orbitals of equal energy singly, with parallel spins, before any pairing occurs. This applies to the three p orbitals and five d orbitals. |
| Pauli exclusion principle | An orbital holds a maximum of two electrons, and paired electrons must have opposite spins. |
Subshell labels combine the energy level with the subshell type: s, p, d or f. Their maximum capacities are 2, 6, 10 and 14 electrons, respectively. These capacities follow from the number of orbitals in each subshell.
Step-by-Step Example: Oxygen
A neutral oxygen atom has atomic number 8, so it contains eight electrons.
- Fill the lowest-energy orbital: .
- Fill the next orbital: .
- Place the remaining four electrons in the three 2p orbitals.
The electron configuration is therefore:
For an orbital diagram, the 2p electrons are arranged as:
Hund's rule requires one electron to enter each 2p orbital before the fourth electron pairs. Pauli's principle requires the paired electrons to have opposite spins.
The superscripts must add to the total number of electrons: . A common misconception is to pair electrons in the first p orbital immediately; this violates Hund's rule because the equal-energy p orbitals must first be singly occupied.
Exam Technique
In Structure 1.3: Electron configurations, check the atomic number, show the correct filling order, and ensure that superscripts total the number of electrons. If asked to draw an orbital diagram, include separate boxes or lines for orbitals and show opposite spins for paired electrons.
