Science & STEM
Molecular Orbital Theory Practice Problems: 15-Question Quiz
This Molecular Orbital Theory quiz helps you practice MO diagrams, find bond order, and spot HOMO and LUMO across 15 multiple-choice questions. Use it to review before class or a test, then connect concepts with molecular geometry practice, explore bigger ideas with a quantum mechanics quiz, or reinforce shapes and hybridization in a vsepr theory quiz.
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1Which molecular orbital is the lowest energy orbital in H2?
2What is the bond order of the H2 molecule according to molecular orbital theory?
3What is the bond order of the He2 molecule?
4Which molecular orbital in H2 is an antibonding orbital?
5In the MO diagram of H2, which orbital is the highest occupied molecular orbital (HOMO)?
6What is the bond order of the O2 molecule based on its molecular orbital electron configuration?
7Which property of O2 arises from its molecular orbital configuration?
8In the molecular orbital diagram of N2, which orbital corresponds to the HOMO?
9What is the bond order of the B2 molecule?
10What is the bond order of the He2+ molecular ion?
11For the O2 molecule, which molecular orbital has lower energy: [@U00CF][@U0192]2p or [@U00CF][@U20AC]2p?
12Which of the following homonuclear diatomic molecules has a bond order of 3?
13How many unpaired electrons are present in F2?
14Identify the lowest unoccupied molecular orbital (LUMO) in the ground state O2 molecule.
15Is the NO molecule paramagnetic or diamagnetic?
16What is the bond order of the peroxide ion O2^2-?
17Which of the following oxygen species has the highest bond order?
18What is the bond order of the cyanide ion CN^-?
19In the molecular orbital diagram for B2, which orbital is the HOMO?
20Which homonuclear diatomic molecule has the smallest HOMO-LUMO gap?
Learning Goals
Learning Outcomes
- Identify molecular orbitals and their symmetry properties.
- Apply MO theory to diatomic molecules and predict bond order.
- Analyze bonding and antibonding interactions for different species.
- Evaluate the stability of molecules using electron configurations.
- Demonstrate understanding of HOMO and LUMO concepts.
Study Guide
Cheat Sheet
- Formation of Molecular Orbitals - Ever wondered how atoms team up in molecules? When atomic orbitals combine, they form molecular orbitals that can be bonding (stabilizing) or antibonding (destabilizing), and the balance between them decides molecular strength. This concept is the foundation for predicting everything from simple diatomics to complex organics. OpenStax: Molecular Orbital Theory
- Constructing MO Diagrams - Drawing molecular orbital diagrams is like creating a roadmap of where electrons hang out in diatomic molecules; it helps you see electron distribution and forecast if a molecule will be magnetic or not. With a clear diagram, you'll predict spin states and bond orders like a pro. Wisconsin OER: MO Theory
- Calculating Bond Order - Bond order = (bonding electrons − antibonding electrons) ÷ 2. This simple formula tells you if a bond is weak, strong, or somewhere in between - hands down one of the quickest ways to assess stability. Mastering this calculation is your ticket to confidently compare molecules. UCalgary Chemistry Textbook
- Interpreting Zero Bond Order - A bond order of zero means no bond forms, so those atoms stay solo artists! As bond order increases, so does bond strength, much like leveling up in a video game. Recognizing this trend helps you predict which molecules can actually exist. OpenStax: Bond Order Insights
- Sigma vs. Pi Orbitals - Sigma (σ) orbitals arise from end-to-end overlap, while pi (π) orbitals come from side-by-side overlap of p orbitals. This difference affects bond strength, shape, and reactivity, so knowing which overlap you have is like having a backstage pass to molecular performance. LibreTexts: σ vs. π Orbitals
- Orbital Filling Rules - Electrons fill MOs by the Aufbau principle (lowest energy first), Hund's rule (maximize unpaired spins), and the Pauli exclusion principle (no two electrons share identical quantum states). It's the same checklist you use for atomic orbitals, just on a molecular scale - think of it as orbital Tetris! UCalgary: MO Filling Rules
- HOMO and LUMO - The Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) are the frontier orbitals that dictate how molecules react and absorb light. Think of HOMO as the electron donor and LUMO as the electron acceptor - crucial players in photochemistry and reactivity! Wikipedia: HOMO & LUMO
- Explaining Paramagnetism - Molecular orbital theory elegantly explains why oxygen (O₂) is paramagnetic: it has two unpaired electrons in antibonding π* orbitals. Valence bond theory can't account for this magnetic behavior, but MO theory does - sparking chemistry plot twists! Purdue ChemEd: Paramagnetism
- Practice with Homonuclear Diatomics - Grab paper and pencil to sketch MO diagrams for molecules like Nâ‚‚, Fâ‚‚, and Bâ‚‚. Repetition will solidify how electrons fill orbitals, reveal bond orders, and explain why some diatomics are more stable or reactive than others. Pearson: Practice MO Diagrams
- MO Theory for Polyatomic Molecules - Beyond diatomics, MO theory shines when exploring delocalized bonding in molecules like benzene or nitrate ions. Understanding these delocalized systems reveals resonance, aromaticity, and electronic transitions in big, complex structures. LibreTexts: Polyatomic MO Theory
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Updated Feb 23, 2026