Final answer:
The three rules for electron configurations are the Aufbau Principle, Hund's Rule, and the Pauli-Exclusion Principle. Hund's Rule can be 'broken' when an atom is ionized or forms a compound.
Step-by-step explanation:
The three rules for electron configurations are:
- Aufbau Principle: Electrons occupy the lowest-energy orbitals possible, starting with 1s and continuing on to higher energy orbitals. Shells increase in energy from 1 to 2 to 3, and so on.
- Hund's Rule: When electrons occupy degenerate orbitals (same shell and subshell), they must first singly occupy each empty orbital before double occupying them. The most stable configuration occurs when the spins of the electrons are parallel.
- Pauli-Exclusion Principle: Each electron has a unique set of quantum numbers and if two electrons are in the same orbital, they must have different spins. For this reason, each orbital can hold a maximum of two electrons.
The rule that can be 'broken' is Hund's Rule. When an atom is ionized or forms a compound, the electron configuration can change and Hund's Rule may be violated.