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Wave Mechanics and Covalent Bond Formation

Ch 9

+2 successful theories approach the formation of a covalent bond Valence Bond Theory

Covalent bond is the overlapping of 2 atomic orbitals when electron spins are paired

AP exam tends to focus on the VB approach

Molecular Orbital Theory Considers that a molecule

is similar to an atom in that both have distinct energy levels that can be populated with electrons

+Valence Bond Theory

2 H atoms approach & interact w/ an overlap of their atomic orbitals with e- that have opposing spins

When the bond forms, the paired e- spread out over the molecule to form the final e- cloud over the nuclei.

Overlap of 2 “s” orbitals – called a SIGMA (σ)BOND

- Every covalent bond has 1 & only 1 σ bond!

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Overlap of an “s” & a “p” orbital

Overlap of 2 “p” orbitals

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Pi (π) bonds Occur if there are double or triple bonds in the molecule Formed by the sideways overlap of 2 “p” orbitals e- density is arranged in the 2 e- clouds, one above and one

below the internuclear axis, so the e- in the π bond do NOT interfere with the σbond

Single bond – 1 σbond

Double bond – 1σbond & 1 π bond

Triple bond - 1σbond & 2 π bonds

Which represents a double bond and which represents a triple bond?

+Hybrid Orbital Model

VSEPR theory explains why certain molecules have the geometric shapes that are measureable in the lab.

Hybrid orbitals describe how the e- might rearrange in a molecule to explain the experimental data on bond angles. Hybrid orbital – a set of orbitals with identical

properties formed from the combination of 2 or more different orbitals with different energies

+sp3 hybrid orbitalsEx. CH4

Structure is tetrahedral, the C has 4 σbonds

e- configuration

1s22s22p2

sp3 hybrid orbital

The “s” and 3 “p” orbitals in C reorganize into 4 sp3 hybrid orbitals. Notice that an sp3 orbital has energy somewhere between the 2s and the 2p

+sp2 hybrid orbitals Ex. CH2O

sp2 hybrid orbitals The C has 3σbond & 1π bond

Structure is trigonal planar.

+sp hybrid orbitals

The C has 2σbonds & 2 π bonds It’s linear

sp hybrid orbital

+Correspondence between hybridization and structure

Structure Hybrid # bonding pairs

# lone pairs

Linear sp 2 0

Trigonal planar sp2 3 0

Bent sp2 2 1

Tetrahedral sp3 4 3

Trigonal pyramidal

sp3 3 1

Bent sp3 2 2

Scientists have NOT been able to demonstrate the existence ofand importance of sp3d and sp3d2 hybrid orbitals and are omitted from the AP Test. Therefore, all other structures will not be shown with their hybridization.