WAEC Chemistry 2019 Theory — Question 12

Model answer

• Aufbau principle: electrons fill orbitals starting from the lowest energy level before occupying higher-energy orbitals. • Pauli exclusion principle: no two electrons in an atom can have the same set of all four quantum numbers (i.e. an orbital can hold a maximum of 2 electrons with opposite spins). • Hund's rule of maximum multiplicity: when filling degenerate orbitals (of the same energy), electrons occupy each orbital singly before any orbital receives a second electron (pairing).