Which rule states that electrons in singly occupied orbitals must have the same spin?

• A. Hund's rule
• B. Aufbau principle
• C. Pauli exclusion principle
• D. Noble gases

Answer: (A)

Hund's rule governs how electrons fill orbitals of the same energy. It says that when you have multiple degenerate orbitals, electrons first occupy those orbitals singly and with parallel (same) spins before any orbital gets a second electron. This arrangement maximizes total spin and stabilizes the atom by increasing exchange energy. So the idea that electrons in singly occupied orbitals must have the same spin is a direct expression of Hund's rule. For example, in a p subshell with three electrons, each electron goes into a different p orbital with the same spin. Only after all three are singly occupied would pairing occur, placing the next electron into one of the orbitals with opposite spin. The Aufbau principle deals with filling order by energy, the Pauli exclusion principle prevents two electrons in the same orbital from having identical quantum numbers (they must have opposite spins if paired), and noble gases are not a rule governing this behavior.