Molecular Orbitals (and MO theory)

Molecular Orbital Theory

a model of chemical bonding in which atomic orbitals combine to form delocalized molecular orbitals

Bonding orbital (Constructive interference)

Results from 2 atomic orbitals added in-phase. It has increased electron density between the nuclei (lower energy and more stable than two separate 1s orbitals)

Antibonding orbital (Destructive interference)

Results from 2 atomic orbitals added out-of-phase (contains a nodal plane that is perpendicular to bonding axis)

Bond order

(# of bonding electrons - # of antibonding electrons) / 2

Strength of bonding orbital

Since an electron is in the middle, there is more attraction between the nuclei which shortens the length and causes a stronger bond (repulsion weakens a bit)

Energies of MO depend on overlap. How?

As distance increases and overlap decreases, the antibonding becomes less antibonding (decreases in energy) and the bonding becomes less bonding (increases in energy) and eventually they become the same

Sigma Bond

is symmetrical about bonding axis (z-axis) and is axially symmetric (s-orbitals

Pi Bond

Has one nodal plane containing bonding axis (p-orbitals)

2p orbitals

can be oriented towards each other or parallel to each other. 2pz is assumed to point along bonding axis while 2px and 2py are aligned perpendicularly.

Parallel 2p orbitals

create pi MOs

Bond type and MO type

Double bond = 1 sigma and 1 pi

Triple bond = 1 sigma and 2 pi

Number of electrons in orbitals

More electrons in a bonding orbital, strengthen the bond while more electrons in antibonding orbital weaken the bond.

Overlap and stabilization

the more overlap between orbitals, the more the bonding orbital is stabilized while the more destabilized the antibonding orbital is

sigma orbitals and nodes

sigma bond has no nodes while antibonding has one that is perpendicular to bind axis

pi orbitals and nodes

pi bonding has one nodal plane which makes the pi bond symmetrical

pi antibonding has the same plane but another node that runs in between the two nuclei and separates them

relationship between bond order and bond strength

greater bond order implies stronger bond and shorter bond length and less likeliness to be paramagnetic

For O and the rest of the periodic table…

sigma MOs are lower in energy than pi MOs (this is because as nuclear charge increases, electrons have incentive to stay close to nuclei)

For N, B, and C

pi orbital is lower in energy than sigma orbital. this is because putting electrons in the pi orbital first prevents shielding that would be from the sigma orbital

How MO energy is affected by electronegativity

The more electronegative an atom is, the lower energy the MO is

Strategy for MO diagram

1) combine electrons from both atoms to create the MO energy ladder

2) place the electrons onto the ladder from lowest to highest energy levels (when orbitals are of the same energy, the electrons must be placed unpaired first then pairing)

Octet rule

there should be 8 electrons around a nonmetal atom for it to be the most stable

delocalized bond

a type of bonding characterized nu a pair of electrons that are spread out over the atoms in a bond, rather than being localized to a single pair of atoms (molecules with overlapping pi bonds would have electron travel the entire length of molecule)

pi orbitals and nodes

lowest energy of pi: no antibonding interactions

next level energy of pi: one antibonding interaction (1 node)

third highest energy of antibonding pi: 2 interactions

highest energy: 3 interactions

sigma symmetry

if you look down the bond axis, you see a circle

pi symmetry

if you look down the bond axis, you see a nodal plan and it looks like a p orbital

radical

a chemical species with an unpaired electron, often very reactive

Exceptions to Octet Rule

1)first period atoms only require 2 electros

2)some stable molecules or ions can have an odd number of valence electrons

3) all central atoms in the third period and lower can have more than eight valence electrons (“expanded octets”)