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Valence Bond Theory

Linus Carl Pauling (February 28, 1901 August 19, 1994) Valence Bond TheoryThe 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.2The combination of electrostatic repulsion and opposing magnetic fields (due to the electrons spin) creates the stability associated with a bonding orbital.Negative chargeElectrostatic repulsionNegative chargeElectron SpinElectron SpinNorth magnetic fieldSouth magnetic fieldMagnetic attractionValence Bond Theory3

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.Valence Bond Theory4

WHY IS THIS OVERLAP NOT POSSIBLE?PROBLEMS WITH LEWIS THEORY

Hybrid OrbitalsIn 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?7

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 slightlyThe 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 orbitalThe new hybrid orbitals are free to become involved in bonds by overlapping with other half filled valence orbitals.

8Hybridization1s2s2pC (z = 6)1s2s2pC (z = 6)1ssp3 C (z = 6)sp3 hybridization1s2s2pC (z = 6)1s2s2pC (z = 6)1ssp2 C (z = 6)sp2 hybridizationp 1ssp C (z = 6)sp hybridizationp p 9Linus Pauling and Hybrid Orbitals

Remember VSEPR

sp3 Hybrid atomic orbitals

12sp3 hybrid orbitals in CH4

sp2 Hybrid atomic orbitals

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sp2 hybrid orbitals in BF3

sp hybrid orbitals in BeF2

sp3d and sp3d2 hybridization

Double & Triple BondsSigma (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.18Double & Triple BondsPi (p) bonds The side-to-side overlapping of half filled p orbitals to make more stable filled bonding orbitals.

19 Double & Triple BondsA 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)20Double & Triple BondsA combination of p and s bonds makes double and triple bonds.Triple bonds 1 Sigma (s) and 2 Pi (p)

Pi (p ) bondPi (p ) bondSigma (s ) bond21