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Quizzes > Quizzes for Business > Education

Ultimate Molecular Orbital Theory Quiz

Explore Electronic Structure and Bonding Concepts

Difficulty: Moderate
Questions: 20
Learning OutcomesStudy Material
Colorful paper art displaying elements of Molecular Orbital Theory for a trivia quiz

Ready to delve into molecular bonding? This Molecular Orbital Theory Quiz offers 15 multiple-choice questions to challenge understanding of orbitals, bond order, and electron interactions. Ideal for chemistry students and educators seeking a concise review, it can be easily customized in our editor to suit specific learning goals. If you enjoy theory challenges, explore our Music Theory Knowledge Quiz or sharpen your reasoning with a Driving Theory Practice Quiz. Visit our quizzes page to discover more practice tests across various topics.

Which molecular orbital is the lowest energy orbital in H2?
σ1s
σ*1s
π1s
δ1s
The σ1s orbital arises from the in-phase combination of two 1s atomic orbitals and is lowest in energy. Antibonding and higher-symmetry orbitals are at higher energies.
What is the bond order of the H2 molecule according to molecular orbital theory?
0.5
1
1.5
2
Bond order is calculated as (number of bonding electrons − number of antibonding electrons)/2 = (2−0)/2 = 1. A bond order of 1 corresponds to a single bond.
What is the bond order of the He2 molecule?
0
1
2
3
He2 has two electrons in the σ1s bonding orbital and two electrons in the σ*1s antibonding orbital, giving (2−2)/2 = 0. Thus, no net bond forms.
Which molecular orbital in H2 is an antibonding orbital?
σ*1s
σ1s
π1s
δ1s
The σ*1s orbital is formed by the out-of-phase combination of 1s atomic orbitals and is antibonding. It is higher in energy than the bonding σ1s orbital.
In the MO diagram of H2, which orbital is the highest occupied molecular orbital (HOMO)?
σ1s
σ*1s
π1s
σ2s
With two electrons in H2, only the σ1s orbital is occupied, making it the HOMO. All higher orbitals remain unoccupied.
What is the bond order of the O2 molecule based on its molecular orbital electron configuration?
1
1.5
2
2.5
O2 has 12 valence electrons: eight occupy bonding MOs and four occupy antibonding MOs. Bond order = (8−4)/2 = 2.
Which property of O2 arises from its molecular orbital configuration?
Diamagnetic
Paramagnetic
Nonmagnetic
Ferromagnetic
O2 has two unpaired electrons in the π*2p antibonding orbitals, which leads to paramagnetism. Diamagnetism would require all electrons to be paired.
In the molecular orbital diagram of N2, which orbital corresponds to the HOMO?
σ2p
π2p
σ*2s
π*2p
For N2, the energy order places σ2p just above π2p, and since it is the highest occupied orbital, it is the HOMO.
What is the bond order of the B2 molecule?
0
1
2
3
B2 has six valence electrons: four in bonding MOs and two in antibonding MOs. Bond order = (4−2)/2 = 1.
What is the bond order of the He2+ molecular ion?
0
0.5
1
1.5
He2+ has three electrons: two in the σ1s bonding orbital and one in the σ*1s antibonding orbital. Bond order = (2−1)/2 = 0.5.
For the O2 molecule, which molecular orbital has lower energy: σ2p or π2p?
σ2p
π2p
σ*2p
π*2p
In O2 and heavier diatomics, the π2p orbitals lie below σ2p due to reduced s - p mixing. Thus π2p is lower in energy than σ2p.
Which of the following homonuclear diatomic molecules has a bond order of 3?
N2
O2
F2
Cl2
N2 has 10 valence electrons filling bonding and antibonding MOs so that bond order = (8−2)/2 = 3, corresponding to a triple bond.
How many unpaired electrons are present in F2?
0
1
2
3
F2 has 14 valence electrons that fill bonding and antibonding MOs with no unpaired electrons, making it diamagnetic.
Identify the lowest unoccupied molecular orbital (LUMO) in the ground state O2 molecule.
σ*2p
π*2p
π2p
σ2p
The π*2p antibonding orbitals are the first unoccupied orbitals in O2, making them the LUMO.
Is the NO molecule paramagnetic or diamagnetic?
Paramagnetic
Diamagnetic
Nonmagnetic
Ferromagnetic
NO has an odd number of electrons, leaving one unpaired electron in the π*2p orbital, which makes it paramagnetic.
What is the bond order of the peroxide ion O2^2-?
1
1.5
2
2.5
O2^2- has 14 valence electrons filling bonding and antibonding MOs such that bond order = (8−6)/2 = 1. Thus the O - O bond is weaker than in O2.
Which of the following oxygen species has the highest bond order?
O2
O2+
O2-
O2^2-
O2+ has one fewer antibonding electron than O2, giving (8−3)/2 = 2.5, which is higher than the bond order of neutral or more reduced oxygen species.
What is the bond order of the cyanide ion CN^-?
2
2.5
3
3.5
CN^- is isoelectronic with N2, having 10 valence electrons that yield eight in bonding and two in antibonding MOs. Bond order = (8−2)/2 = 3.
In the molecular orbital diagram for B2, which orbital is the HOMO?
σ2p
π2p
σ*2s
π*2p
B2 has six valence electrons, and after filling σ2s and σ*2s, the remaining two occupy the degenerate π2p orbitals, making π2p the HOMO.
Which homonuclear diatomic molecule has the smallest HOMO-LUMO gap?
B2
C2
N2
O2
B2 has fewer valence electrons and incomplete filling of π2p orbitals, resulting in a smaller gap between the HOMO (π2p) and the LUMO (σ2p) than in heavier diatomics.
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Learning Outcomes

  1. Identify molecular orbitals and their symmetry properties.
  2. Apply MO theory to diatomic molecules and predict bond order.
  3. Analyze bonding and antibonding interactions for different species.
  4. Evaluate the stability of molecules using electron configurations.
  5. Demonstrate understanding of HOMO and LUMO concepts.

Cheat Sheet

  1. 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
  2. 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
  3. 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
  4. 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
  5. 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
  6. 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
  7. 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
  8. 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
  9. Practice with Homonuclear Diatomics - Grab paper and pencil to sketch MO diagrams for molecules like N₂, F₂, and B₂. 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
  10. 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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