Which of the following must be observed when filling a molecular orbital energy diagram?

• A. Electrons can be placed in any orbital regardless of energy.
• B. Electrons must always pair up in each orbital before occupying separate orbitals.
• C. Only bonding orbitals are filled; antibonding orbitals remain empty.
• D. Electrons must fill the lowest energy orbitals first before occupying higher energy orbitals.

Answer: (D)

Verified step by step guidance

1. Understand that when filling molecular orbital (MO) diagrams, electrons occupy orbitals according to the Aufbau principle, which states that electrons fill the lowest energy orbitals first before moving to higher energy orbitals.
2. Recall Hund's rule, which says that electrons will fill degenerate orbitals (orbitals of the same energy) singly first, with parallel spins, before pairing up in any one orbital.
3. Recognize that electrons do not simply fill any orbital regardless of energy; energy order must be followed to correctly represent the molecule's electronic structure.
4. Note that both bonding and antibonding orbitals can be occupied depending on the number of electrons; antibonding orbitals are not always empty.
5. Summarize that the key rule to observe is that electrons must fill the lowest energy orbitals first, ensuring the most stable electronic configuration in the molecular orbital diagram.