H3 Chemistry 9813 · Study focus: H3 Chemistry: Apply LCAO to homonuclear diatomic molecules
H3 Chemistry: Apply LCAO to homonuclear diatomic molecules
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Apply LCAO to homonuclear diatomic molecules
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Orientation and prerequisite retrieval
Combining two compatible AOs produces two MOs: one bonding and one antibonding.
The in-phase linear combination is lower in energy for a bonding interaction.
Definitions and exact language
The out-of-phase linear combination has an additional internuclear node.
Only AOs of compatible symmetry and comparable energy mix substantially.
Two H 1s AOs form σ1s and σ*1s molecular orbitals.
Two bond-axis 2p orbitals form σ2p and σ*2p partners.
Text alternative: Text alternative: Two parallel off-axis p orbitals form π2p and π*2p partners. A complete LCAO sketch aligns parent AOs, phases, resulting MOs and energy.
Detailed molecular explanation
Two H 1s AOs form σ1s and σ*1s molecular orbitals.
Two bond-axis 2p orbitals form σ2p and σ*2p partners.
Worked leaf-specific case
Two parallel off-axis p orbitals form π2p and π*2p partners.
AO coefficients show relative contribution and phase in a linear combination.
- A candidate writes, “Two compatible atomic orbitals make only one molecular orbital.” Which correction should replace it?
Open the feedback checkpoint after attempting
- Combining two compatible AOs produces two MOs: one bonding and one antibonding.
Guided evidence check
Homonuclear partners contribute equally by symmetry to each two-centre MO.
The number of basis AOs is conserved as the number of resulting MOs.
- Which labelled diagram, spectrum or calculation would most directly disprove “Any pair of AOs mixes equally regardless of symmetry or energy.”?
Open the feedback checkpoint after attempting
- Only AOs of compatible symmetry and comparable energy mix substantially.
Independent transfer
Bonding and antibonding levels lie below and above the interacting AO reference in the qualitative model.
A complete LCAO sketch aligns parent AOs, phases, resulting MOs and energy.
- Design a specific chemical check that would expose the error in “Parallel off-axis p orbitals form σ symmetry.”
Open the feedback checkpoint after attempting
- Two parallel off-axis p orbitals form π2p and π*2p partners.
Misconception repair
Misconception: Both combinations must lie below the parent AO energies.
Repair: Bonding and antibonding levels lie below and above the interacting AO reference in the qualitative model.
- Write leaf-specific feedback for the misconception “Both combinations must lie below the parent AO energies.”
Open the feedback checkpoint after attempting
- Bonding and antibonding levels lie below and above the interacting AO reference in the qualitative model.
Practice, unseen assessment, re-test and next step
Complete the diagnostic questions, review the feedback, then attempt the final check.
After delayed re-test, continue to Apply LCAO to benzene and linear polyenes at /learning/h3-molecular-orbitals-lcao-conjugated-lesson.html.
- Review the feedback for each diagnostic question, then attempt the final check.
Open the feedback checkpoint after attempting
- Before moving on, state this boundary: A complete LCAO sketch aligns parent AOs, phases, resulting MOs and energy.