Transcript Document
Molecular Orbitals: combine atomic orbitals (AOs) from all the atoms in a molecule
into the same number of molecular orbitals.
MOs have different shapes, sizes, and energies, and have two electrons max. per
orbital (Pauli Exclusion Principle).
Wave functions Ψ (+/-) not the probability clouds combined into MOs.
Ex: 2 ways of combining two 1s atomic orbitals in the hydrogenlike molecules (same
or opposite signs. Equivalent to adding or subtracting Ψ.
Add (Ψ with same sign), MO has electron density between the nuclei. (attract nuclei,
shield nuclear repulsion). electron-pair bond Bonding orbital.
Subtract (Ψ with opposite sign), MO has no electron density (node) between the
nuclei. (no electrons to attract nuclei or shield nuclear repulsion); pulls a molecule
apart, an idea that could not be accounted for with the Lewis theory. Antibonding
orbital.
If a molecular orbital is completely symmetrical about the line connecting the nuclei,
it is called a s (bonding or antibonding) orbital. If electron density is above/below
line, it’s called p (bonding or antibonding) orbital
Lone pairs in Lewis model contribute nothing to bonding, correspond to the selfcanceling bonding and antibonding orbitals in MO theory. Both theories say that nonbonded electron pairs make no contribution to bonding, but the Lewis model assigns
pair to individual atoms, whereas MO theory states that both are shared by the entire
molecule. For diatomic molecules in Pd 2, Lewis models show all electrons are
paired, whereas with MO theory, B and 0 each have two unpaired electrons in p
orbitals of the same energy
Combine AO as follows:
1. Two AO's must be close enough in space to overlap appreciably before they can
be combined into an MO.
2. The combining AO's must be of similar energy.
3. The orbitals must have the same symmetry around the bond axis.
Ex: HF- Increased nuclear charge of +9 on F, compared to +1 on H, pulls the
electrons in all the fluorine quantum levels and makes them more stable (lower
energy). Outer occupied orbital (2p) in F and the 1s H orbital are the ones that are of
similar energy. The first ionization energy (Eion1) of hydrogen (1s) is 313 kcal/mole,
fluorine (2p) is 402 kcal mole. Therefore, 2p orbitals in F is lower in energy than the
1s orbital in H. These are the orbitals that are similar enough in energy to combine.
y = yA - yB
MOLECULAR ORBITALS
+
energy
-
+ -
s orbital
s orbital
y = y A + yB
resultant
++
+
energy
+
s2p*
energy
p2p*
2p
2p
s2p
p2p*
2p
2p
p2p
s2p
p2p
s2s*
2s
s2p*
s2s*
2s
s2s
MO DIAGRAM: homonuclear diatomic
Molecules = Li2 to N2
2s
2s
s2s
MO DIAGRAM: homonuclear diatomic
Molecules = O2 to F2
Fill out the MO diagrams for:
1s
1s
1s
1s
He2
H2
s2p*
s2p*
p2p*
2p
2p
p2p*
2p
s2p
2p
p2p*
2p
s2p
p2p
p2p
s2p
p2p
s2s*
2s
s2p*
s2s*
2s
2s
s2s*
2s
2s
2s
s2s
s2s
Li2
N2
O2
2s
p2p
s2s
s2s*
2s
2p
s2p
s2p*
p2p*
2p
s2s
F2
2p
MOLECULAR ORBITALS: COVALENT AND IONIC BONDING
s*
H
H
H2
nonpolar covalent
EN H = EN H
bonding and antibonding MOs same
contribution from both atoms.
Pure covalent bond
s
s*
H
EN F > EN H
E Bonding MO more like E F2p AO
E antibonding MO more like E H 1s AO
form polar covalent bond (partial ionic bond)
F
HF
polar covalent
s
s*
Li
EN F >> EN Li
E Bonding MO ~ E F2p AO
Bonding pair essentially given to F,
form ionic bond
LiF
ionic
F
s