Molecular Orbitals
High-Yield Summary
- Molecular orbitals form when atomic orbitals interact via constructive interference (same-sign wave functions → bonding MO, lower energy/stable) or destructive interference (opposite-sign → antibonding MO, higher energy/destabilizing).
- Only electrons in bonding orbitals stabilize a molecule; electrons in antibonding orbitals weaken or prevent bonding.
- Sigma (σ) bonds form from direct head-to-head overlap along the internuclear axis — the strongest covalent bond type; can form via s-s, s-p, p-p, or hybrid orbital overlap.
- Pi (π) bonds form from sideways overlap of parallel p orbitals — weaker than sigma, and can't exist without an accompanying sigma bond.
- Bond composition: single bond = 1σ; double bond = 1σ + 1π; triple bond = 1σ + 2π.
Bond Strength and Length by Bond Order
| Bond type | Composition / strength & length |
|---|---|
| Single (C–C) | 1σ — longest, weakest |
| Double (C=C) | 1σ + 1π — shorter, stronger |
| Triple (C≡C) | 1σ + 2π — shortest, strongest |
Key Terms
- Bonding molecular orbital
- Lower-energy, stabilizing orbital produced by constructive interference of atomic orbital wave functions.
- Antibonding molecular orbital
- Higher-energy, destabilizing orbital produced by destructive interference of atomic orbital wave functions.
- Internuclear axis
- Straight line connecting the nuclei of two bonded atoms; sigma bonds overlap directly along it.
- Sigma (σ) bond
- Strongest covalent bond; head-to-head orbital overlap along the internuclear axis. Every single, double, and triple bond contains exactly one.
- Pi (π) bond
- Weaker bond from sideways overlap of parallel p orbitals, above/below the bond axis; requires a pre-existing sigma bond framework.
Common MCAT Trap
- A pi bond can never exist alone — every double or triple bond still contains exactly one sigma bond plus the pi bond(s), not two/three pi bonds.
- MO diagrams (bond order, magnetic properties) aren't typically MCAT-tested — the underlying bonding/antibonding concept is what matters.
- More bonds ≠ simply 'stronger per bond' — the sigma bond alone is strongest per-overlap; added pi bonds increase total bond strength while shortening bond length, not the other way around.
Quick Recall
What type of interference produces a bonding MO vs. an antibonding MO?
How many sigma and pi bonds make up a triple bond?
Why is a sigma bond stronger than a pi bond?
Rank single, double, and triple carbon-carbon bonds by length.