Hund's rule states that electrons will occupy degenerate orbitals (orbitals of the same energy level) singly before pairing up. In the given options: - Option (1) obeys Hund's rule: all three orbitals are fully paired, following the principle.
- Option (2) also obeys Hund's rule: the two electrons in the first orbital are placed singly, then the second pair is formed.
- Option (4) follows Hund's rule as well, as it places electrons singly in each orbital before pairing. However, option (3) does not obey Hund's rule because the second electron should have been placed singly in the available orbital rather than pairing up with the existing electron. According to Hund’s rule, the second electron should occupy a different orbital instead of pairing immediately. Thus, the correct answer is option (3).
