Molecular Orbitals Flashcards
As overlap of atomic orbitals increases…
- bonding MO energy is lowered (stabilised more)
- antibonding MO energy is raised (destabilised more)
Significant overlap/formation of a MO only occurs between atomic orbitals with
- similar size
- similar energies
- correct symmetry
Aufbau Principle
Pauli’s Exclusion Principle
Hund’s Rule
Key Features: Bonding MO
- Overlap of atomic orbital wavefunctions is in phase
- Increase in the electron density between the nuclei
- E(Bonding MO) < E(AO from which it is formed)
Key Features: Antibonding MO
- Overlap of atomic orbital wavefunctions is out-of-phase
- Decrease in the electron density between the nuclei
- E(Anti-bonding MO) > E(AO from which it is formed)
Within a heteronuclear diatomic, the two atoms are from different atoms (e.g LiH):
- AO do NOT have the same energy in each atom
- Need to work out the relative energies of the AO on each atom
- Bonding MO – bigger contribution from more electronegative atom
- Anti-bonding MO – bigger contribution from less electronegative atom
Linear Combination of Atomic Orbitals (LCAO)
ψMO = c1 φAO(A) + c2 φAO(B)
( + for in-phase, - for out-of-phase)
The more electronegative the element…
The lower the AO energies
Effect of electronegativity on bonding of MO
Bonding MO: skewed towards the more electronegative atom
Anti-bonding MO: skewed towards the less electronegative atom
s-p mixing in heteronuclear diatomics
more likely to occur as the s and p orbitals are similar energies
MO Theory
- Ground & excited states
- Quantitative picture of bonding
- Delocalised MOs for electrons (can be spread over whole molecule)
- Only theory to explain behaviour of O2 (paramagnetic) & NO (radical)
VB Theory
- Ground state
- Qualitative picture of bonding & molecular structure
- Localised bond + e- pairs
- Quick “picture” of geometry
Bonding MO
- Lower in energy than the AO from which it is formed
- Increase in electron density between the nuclei
- Overlap of atomic wavefunction is in phase or constructive
