AP Notes Chapter 9 Hybridization and the Localized Electron Model Valence Bond Theory Molecular Orbital Theory Metals & Semiconductors

Hybridization and the Localized Electron Model Localized Electron Model developed from Valence Bond Theory

Why do we need it? Consider the water species. H 1s1 O 1s2 2s2 2p4 Gives 2 H’s with no e- and O with full octet. 1s ___ 1s ___ 2s ___ 2p ___ ___ ___

We Get H 1s1 O 1s2 2s2 2p4 Gives 2 H’s with no e- and O with full octet. 1s ___ 1s ___ 2s ___ 2p ___ ___ ___

Hybridization Process that changes properties of valence electrons by mixing atomic orbitals to form special orbitals for bonding atomic molecular orbitals orbitals AO MO

Principles 1. Conservation of orbitals 2. Hybrid correlates with molecular geometry 3. Energy level of MO is between that of AO’s 4. All bonded atoms hybridize

WHEN ATOMS BOND atomic orbital hybrid orbital

All hybrid orbitals of an atom are said to be DEGENERATE (of equal energy)

CH4 C: AO 2p __ __ __ 2s ____ H C H H H

CH4 MO __ __ __ __ H C H H H sp3 hybrid orbitals

4 Items Equally Distributed sp3 hybridization sp3 hybrid orbitals tetrahedral species sp3 shape tetragonal 4 Items Equally Distributed

Tetragonal

C C H H H H H H H H Lewis Structure Electron Pair Geometry Molecular Model C H H H H C H H H

InCl3 In: AO 5p __ __ __ 5s _____

InCl3 MO __ __ __ Cl In 5p __ Cl Cl sp2 hybrid orbitals

trigonal planar species 3 Items Equally Distributed sp2 hybridization sp2 hybrid trigonal planar species sp2 shape 3 Items Equally Distributed

BaCl2 Cl - Ba - Cl Ba: AO 6p ___ ___ ___ 6s _____

BaCl2 Cl - Ba - Cl Ba: MO 6p ___ ___ sp hybrid orbitals

2 Items Equally Distributed sp hybridization sp hybrid linear species sp shape 2 Items Equally Distributed

PF5 P: AO 3d ___ ___ ___ ___ ___ 3p ___ ___ ___ 3s ____

PF5 P: MO 3d ___ ___ ___ ___ ___ ___ ___ ___ ___ sp3d hybrid orbitals

5 Items Equally Distributed sp3d hybridization sp3d shape trigonal bipyramid species 5 Items Equally Distributed

SF6 S: AO 3d ___ ___ ___ ___ ___ 3p _____ ___ ___ 3s _____

SF6 S: MO 3d ___ ___ ___ ___ ___ ___ ___ ___ ___ sp3d2 hybrid orbitals

6 Items Equally Distributed sp3d2 hybridization sp3d2 shape octahedral species 6 Items Equally Distributed

Multiple Bonds sigma bonds () pi bonds ()

EXAMPLES O2 2 p-orbitals touching end to end sigma - σ O 1s2 2s2 2p4 2p4 2s2 1s2 O 2 p-orbitals touching end to end sigma - σ 2p-electrons reaching over and under pi - π 2s ___ 2p ___ ___ ___ 2p ___ ___ ___ 2s ___ ___ ___

Valence Bond Theory Multiple Bond Examples C2H4 (ethylene) s (sp3 hybridization) p ( bonding) both

EXAMPLES C2H2

EXAMPLES CH3COOH

MOLECULAR ORBITAL MODEL Valence Bond Theory concentrates on individual bonds in a molecule and tends to ignore electrons not used in bonding.

Molecular Orbital Theory assumes ALL the orbitals of the atoms are able to take part in bonding.

Every atom has a complete set of orbitals, but not all of them contain electrons

Remember that orbitals are really the solutions of Schrodinger’s equation, and that they are called wave-functions

1s wavefunction r

- + Negative here Positive 2pz wavefunction

2pz orbital - + 1s orbital

While wave functions can be positive or negative, probabilities can only be positive.

Wave functions, like waves, can overlap with one another Wave functions, like waves, can overlap with one another. They can reinforce each other, or they can cancel each other out.

. + plus 1sA 1sB B A A sigma, s, bonding orbital

. + - minus 1sA 1sB B A A sigma star, s*, anti-bonding orbital

The work on molecular orbitals can be generalized to p-orbitals.

2pz s2p A s2p bonding orbital

2pz s2p* A s2p* antibonding orbital

plus A B 2py p2p A p2p bonding orbital

minus A B 2py p2p* A p2p* antibonding orbital

Many combinations of orbitals can produce bonding and anti-bonding molecular orbitals, s with p, d with p, etc.

Orbitals on the two bonding atoms must meet 2 conditions They must be similar in energy They must have the right symmetry

plus 2pz 2pz 2py 2py Orbitals pointing in different directions cannot overlap to form molecular orbitals.

Molecular Orbital Theory 1. Molecular orbitals are made from atomic orbitals 2. Orbitals are conserved 3. Molecular orbitals form in pairs: bonding & antibonding

Bonding Molecular Orbital Geometry favorable to overlap

When a bonding orbital is formed, the energy of the orbital is lower than those of its parent atomic orbitals.

Anti-bonding Molecular Orbital Geometry not favorable to overlap

Similarly, when an anti-bonding orbital is formed, the energy of the orbital is higher than those of its parent atomic orbitals.

Molecular Orbital Diagrams Bond Order

Examine some homonuclear diatomic molecules Hydrogen Helium

s2s* 1sA 1sB s2s

Paramagnetic 1. Responds to magnetic field 2. Has unpaired electrons

Diamagnetic 1. Does not respond to magnetic field 2. All electron paired

1s 2s 2px 2py 2pz s1s s1s* s2s s2s* s2p p2p s2p* p2p* fluorine gas

1s 2s 2px 2py 2pz s1s s1s* s2s s2s* s2p p2p s2p* p2p* oxygen gas

Using MO Theory, molecules have an electron configuration Oxygen gas (s1s)2(s1s*)2 (s2s)2(s2s*)2 (p2py)2 (p2py*)2 (s2pz)2 (p2px)2

nitrogen gas 1s 2s 2px 2py 2pz s1s s1s* s2s s2s* s2p p2p s2p* p2p* Magnet Movie

Bond Strength Bond Length

Strengths of Localized Electron Model 1. Simple 2. Easy to understand 3. Predicts geometry of molecule

Limitations of Localized Electron Model 1. Does not address concept of resonance or unpaired e- 2. Cannot explain color in transition metal compounds

Strengths of Molecular Orbital Model 1. Better represents actual molecular system 2. Provides basis for explaining properties of molecular systems

Limitations of Molecular Orbital Model 1. MO diagrams are complex. 2. MO diagrams are difficult for molecules with more than two atoms. 3. No prediction of geometry

Combining the Localized Electron and Molecular Orbital Models

Draw the Lewis structure of benzene C6H6 Lewis Structure

C2H4 + Br2  C2H4Br2 C6H6 + Br2  NR

s bonds in benzene p bonds in benzene benzene

Isomerism Isomers – two or more compounds with same molecular formula but different arrangements of atoms Cis – Trans Isomerism (NOT mirror images of each other NOT super imposable. Cis Trans

Resonance and MO X X The more resonance structures the more stable the molecule

Metals & Semiconductors Read pg657-669 Study Figures 1-23 Know Insulators Conductors, Semiconductors – intrinsic, extrinsic Dopants