Humber Admissions Practice Test

What does Hund's rule specify regarding electron occupation in sub shells?

1. All orbitals must be filled before any is paired

2. Electrons are distributed randomly within a sub shell

3. Each orbital must contain two electrons

4. Each singly occupied orbital must have opposite spins

The correct answer is: All orbitals must be filled before any is paired

Hund's rule states that when electrons occupy orbitals of the same energy (degenerate orbitals), they will first fill each orbital singly before any orbital gets a second electron. This means that all orbitals within a sub shell must be filled with one electron each before electrons begin to pair up in those orbitals. This rule is significant because it minimizes the repulsion between electrons, as they can remain as far apart as possible when they are in different orbitals. The first option reflects this idea accurately, highlighting the necessity of single occupancy in each orbital before any pairing happens. The alternative choices do not accurately represent Hund's rule; for instance, the suggestion that electrons are distributed randomly ignores the systematic approach Hund's rule promotes. Similarly, the idea that each orbital must contain two electrons contradicts the essence of the rule, as it emphasizes the importance of single occupancy first. Lastly, while opposite spins for electrons in paired orbitals is a valid concept related to Pauli's exclusion principle, it does not encompass the definition or application of Hund's rule specifically.