When filling up the orbitals with electrons, they follow the following 3 rules.
- Aufbau Principle
- Hund’s Rule
- Pauli Exclusion Principle
We will use the analogy of passengers on a bus to explain these 3 rules.
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All passengers would prefer to fill up all the seats on the lower deck entirely before going to a higher deck.
Similarly, electrons would prefer to fill up orbitals of a lower energy first before going to a higher energy orbital. The order in which the orbitals are filled is as follows.
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All passengers would prefer to sit alone rather than share their seats.
Similary, electrons would prefer to occupy orbitals singly first before attempting to pair up.
Pauli Exclusion Principle
When all unoccupied seats are taken, passengers would then be forced to pair up. Being heterosexual beings, they would prefer to pair up with another passenger of an opposite gender.
Similarly, electrons would pair up with opposite spins.
Next, we will examine how ionisation energies vary in the periodic table.