Humber Admissions Practice Test

Which rule states that all orbitals of a sub shell must be singly occupied before any is doubly occupied?

1. Pauli Exclusion Principle

2. Aufbau Principle

3. Hund's rule

4. Covalent Theory

The correct answer is: Hund's rule

Hund's rule is the concept that governs the behavior of electrons when they fill orbitals within a sub shell. According to this rule, when multiple orbitals of the same energy level (degenerate orbitals) are available within a sub shell, electrons will first occupy each orbital singly before any orbital receives a second electron. This is crucial for minimizing electron-electron repulsion and maximizing the total spin, which results in a more stable arrangement. In the context of atomic structure, this principle explains why, for example, in the p, d, or f sub shells, electrons will spread out to occupy different orbitals to achieve a lower energy state. Understanding this rule is essential for predicting the electronic configuration of an atom and consequent chemical properties. Therefore, knowing Hund's rule helps in grasping not just individual electron arrangements, but also the reasoning behind the stability of certain atomic configurations.