Molecular Orbital Theory

Molecular Orbitals

Just as atomic orbitals belong to a particular atom, a molecular orbital belongs to molecules as a whole

Each orbital is filled with 2 electrons

There are 2 types of bonding orbitals: sigma and pi

Sigma Bond ()

A Sigma Bond is when 2 atomic orbitals combine to form a molecular orbital that is symmetrical around the axis

S orbitals overlapping P orbitals overlapping end-to-end

Pi Bond ()

Pi bonding electrons are likely to be found in a loop-shape above and below the axis

Pi bonds are weaker than sigma bonds because they overlap less

P orbitals overlapping side-by-side

 

BONDING

ANTIBONDING

Since orbitals are waves of electrons, they can combine either constructively (forming a bonding molecular orbital), or destructively (forming an antibonding* molecular orbital).

MO Theory Molecular Orbitals (MO’s) from Atomic Orbitals (AO’s) 1. # of Molecular Orbitals = # of Atomic Orbitals

2. The number of electrons occupying the Molecular orbitals is equal to the sum of the valence electrons on the constituent atoms.

3. When filling MO’s the Pauli Exclusion Principle Applies (2 electrons per Molecular Orbital) and Aufbau’s principle Applies (start at the lowest energy)

4. For equal energy MO’s, Hund's rule applies.

Bond Order To find bond order add total electrons in the

bonding orbitals and subtract total electrons in antibonding orbitals, then divide by 2

Example: Li2

MO Diagram for O2

http://www.chem.uncc.edu/faculty/murphy/1251/slides/C19b/sld027.htm

Molecular Orbital Diagram (HF)