Molecular Orbitals

We have been a little simplistic in our

analysis of orbitals

Which is better, a Lewis structure or

accounting for each electron?

Unpaired electrons result in a molecule being attracted to a magnwtic field, O₂ is attracted to a magnetic field – paramagnetic

No unpaired electrons – diamagnetic,

weakly repelled from magnetic field

Can we account for unpaired electrons in

O₂?

Molecular orbitals have similar

characteristic to atomic orbitals

What is an atomic orbital?

We construct MO’s from LCAO’s

We combine atomic orbitals from the

atoms in the molecule

Consider the two 1s orbitals of H – we

can construct two molecular orbitals for H₂

One is Bonding (s_1s),

one is Antibonding (s_1s*)

Bond order = ½ (# bonding e –

# antibonding e)

BO = 1, single bond

BO = 2, double bond

BO = 0, no bond

What is the bond order for H₂?

What is the bond order of H₂+?

What is the bond order of H₂-?

What happens to the bond distance of H-

H if I add or subtract an electron?

Which molecule would we expect to be

able to make, He₂ or He₂+?

Rules for constructing MO’s

1. # of MO’s = # atomic orbitals

2. only atomic orbitals of similar energies combine

3. each MO can accommodate 2 electrons (PEP)

4. When MO’s have the same energy, one electron enters each (Hund’s Rule)

5. The ability of 2 atomic orbitals to make MO’s is related to their spatial overlap (p_x’s will mix, but not p_x and p_y why?)

Li₂, Be₂, B₂, N₂