Valence Bond Theory Linus Carl Pauling (February 28, 1901 – August 19, 1994)

Valence Bond Theory The half filled orbital of one atom overlaps a half filled orbital of a second atom to form a full orbital with two electrons spinning in opposite directions. The bonding atoms arrange themselves in order to maximize the overlap of the half-filled orbitals. Maximum overlapping of the orbitals creates a bonding orbital with a lower energy and increased stability.

Valence Bond Theory The combination of electrostatic repulsion and opposing magnetic fields (due to the electron’s spin) creates the stability associated with a bonding orbital. Electron Spin Negative charge Electrostatic repulsion Negative charge Electron Spin “North” magnetic field Magnetic attraction “South” magnetic field

Valence Bond Theory During this process, two atoms approach each other and allow their half filled orbitals to overlap and form the stability of a filled bonding orbital.

PROBLEMS WITH LEWIS THEORY

Hybrid Orbitals In some situations a more advantageous bonding scenario can be established by promoting electrons from a full orbital to a similar empty orbital to create two half filled orbitals that are available for bonding. The resulting orbital is a mixture of the two original orbitals and is called a hybrid orbital. For eg. What is the simplified energy level diagram (ie. Valence electrons) for Carbon?

Each hybrid orbital is called an “sp3” orbital One s e- is promoted to the empty p orbital This causes the energy of all 4 orbitals to shift: The energy of the p orbitals is reduced slightly The energy of the s orbital is increased slightly Each orbital is a hybrid of 1 “s” and 3 “p” orbitals Each hybrid orbital is called an “sp3” orbital The new hybrid orbitals are free to become involved in bonds by overlapping with other half filled valence orbitals.

Hybridization C (z = 6) C (z = 6) C (z = 6) C (z = 6) C (z = 6) 1s 2s 2p C (z = 6) 1s sp3 C (z = 6) sp3 hybridization 1s 2s 2p C (z = 6) 1s sp2 C (z = 6) sp2 hybridization p 1s 2s 2p C (z = 6) 1s sp C (z = 6) sp hybridization p 1s 2s 2p C (z = 6)

Linus Pauling and Hybrid Orbitals

Remember VSEPR…

sp3 Hybrid atomic orbitals

sp3 hybrid orbitals in CH4

sp2 Hybrid atomic orbitals

sp2 hybrid orbitals in BF3

sp hybrid orbitals in BeF2

sp3d and sp3d2 hybridization

Double & Triple Bonds Sigma (s) bonds – The “end-to-end” overlapping of half filled orbitals to make a full bonding orbital of lower energy level (i.e. – more stable) They occur between “s”, “p” and hybrid orbitals (“sp”,“sp2” & “sp3”) to make single covalent bonds.

Double & Triple Bonds Pi (p) bonds – The “side-to-side” overlapping of half filled “p” orbitals to make more stable filled bonding orbitals.

Double & Triple Bonds A combination of “p” and “s” bonds makes double and triple bonds. Single bonds Sigma only (s) Double bonds 1 Sigma (s) and 1 Pi (p)

Triple bonds 1 Sigma (s) and 2 Pi (p) Double & Triple Bonds A combination of “p” and “s” bonds makes double and triple bonds. Sigma (s ) bond Pi (p ) bond Pi (p ) bond Triple bonds 1 Sigma (s) and 2 Pi (p)