MO diagram for homonuclear diatomic molecules Li2 through N2

MO diagram for homonuclear diatomic molecules O2 and F2

N2

Lewis diagram :NN:

each atom supplies 5 electrons; a total of 10 electrons must be assigned

N2: 2s2 *2s

2 2p4 2p

2

Bond order for N2

= 0.5 (8 - 2) = 3

O2

Molecular Orbital Theory

O2: 2s2 *2s

2 2p2 2p

4 *2p1 *2p

1

Unpaired electrons

- paramagnetic

Bond order = 2

Agrees with Lewis diagram

O O Lewis model

Heteronuclear diatomics - electrons are unequally shared by the two atoms due to electronegativity differences.

= cA A + cB B

Where cA and cB are coefficients which indicate the contribution of each AO in the MO

In a homonuclear molecule: cA = cB = 1

MO energy-level diagram for a AB type molecule.

A is more electronegative than B

Energy levels of the more electronegative element is lower than the less electronegative element

: higher contribution from A2s

: higher contribution from B2s

CO - ten valence electrons

CO: 2s2 *2s

2 2p4 2p

2

Bond order = 0.5 (8 - 2) = 3

Highest occupied MO (HOMO)

Lowest unoccupied MO (LUMO)

Polyatomics - same principles as diatomics except that the MOs spread all over the atoms in the molecule

An electron pair helps to bind the whole molecule,

not just a pair of atoms.

H2O: six atomic orbitals (one O2s, three O2p, and two H1s)

These six orbitals are used to build six molecular orbitals in which the degree of net bonding characters is related to the number of internuclear nodes.

Molecular Orbitals of H2O

http://www.shef.ac.uk/chemistry/orbitron/

http://www.whfreeman.com/chemicalprinciples/con_index.htm?03

Spectroscopy

Spectroscopy: interaction of light with matter

Average Bond energies (kJ/mol)

C-H: 413 C=C: 610 H-F: 565

H-H: 436 CC: 835