Molecular Geometry and Hybridization of Atomic Orbitals

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Chemical Bonding II:Molecular Geometry and

Hybridization of Atomic OrbitalsChapter 10

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Chapter 10; Molecular Structure

I. Molecular Structure (3-D Shape) of Molecules Following Octet Rulea) VSEPR Theory

i. Regions of High Electron Densityii. 3-D Arrangement

II. Polarity of MoleculesIII. Solubility

Chapter 10; Molecular Structure

IV. Valence Bond Theorya) Hybrid Orbitalsβ) σ vs. Π bonds

V. Expanded Octets and Molecular Structure

Valence shell electron pair repulsion (VSEPR) model:

Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs.

10.1

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Predicting Molecular Structure

A. Diatomic Molecules (N2, Cl2, ICl)i. All all linear since 2 points define a line

B. Polyatomic Moleculesi. Draw Lewis Dot Structureii. Determine Regions of High Electron Density

(HED) around central atom in Lewis Structure.

Regions of High Electron Density

• Chemical Bonds• Single, double, triple bonds all count as 1 region

• Lone Pair• Free Radical

Predicting Molecular Structure(cont)

iii. Choose 3-D arrangement that places regions of High Electron Density (HED) as far apart as possible

AB2 2 0 linear linear

Class

# of atomsbonded to

central atom

# lonePairs/fr on

central atomArrangement ofelectron pairs

MolecularGeometry

VSEPR

AB3 3 0 trigonalplanar

trigonalplanar

10.1

AB4 4 0 tetrahedral tetrahedral

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Cl ClBe

2 atoms bonded to central atom0 lone pairs on central atom 10.1 10.1

10.1

NO2 vs CO2

Class Molecular Structure

Bond Angle

NO2 AB2E Bent, Angular

< 120°

CO2 AB2 Linear 180°

C OO

N

O O

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BH3 vs NH3

Class Molecular Structure

Bond Angle

BH3 AB3 TrigonalPlanar

120°

NH3 AB3E TrigonalPyramidal

<109.5°

B

HH

H

NH

HH

Effect of Lone Pair on Central AtomTotal – 3 HED

Class

# of atomsbonded to

central atom

# lonePairs/fr

on central atom

Arrangement ofelectron pairs

MolecularGeometry


trigonalplanar

AB2E 2 1 trigonalplanar bent

Effect of Lone Pair on Central AtomTotal – 4 HED

Class

# of atomsbonded to

central atom

# lonepairs on


MolecularGeometry


AB3E 3 1 tetrahedral trigonalpyramidal

AB2E2 2 2 tetrahedral bent

HO

H

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Molecular Structure Depends on # Atoms and # HED

2 3 4

2 Linear, 180°

Angular (Bent), <120°

Angular (Bent), <109.5°

3 Trigonal Planar, 120°

Trigonal Pyramidal, 109.5°

4 Tetrahedral, 109.5°

Regions of HED around Central Atom

# A

tom

s S

urro

undi

ng C

entra

l Ato

m

Dipole Moments and Polar Molecules

10.2

H F

electron richregionelectron poor

region

δ+ δ−

10.2

Determining Polarity of Molecules

1) Draw Lewis Dot Structure2) Determine Molecular Structure (3-D

Shape)3) Look for Dipoles Within Molecule4) Determine if Dipoles Reinforce or Cancel

• Reinforce => Polar Molecule• Cancel => Nonpolar Molecule

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10.2

Which of the following molecules have a dipole moment?H2O, CO2, SO2, and CH4

O HHdipole momentpolar molecule

SO O

CO O

no dipole momentnonpolar molecule

dipole momentpolar molecule

C

H

H

HH

no dipole momentnonpolar molecule

Solubility

• “Like” dissolves “Like– Polar substance dissolves in Polar Substance– NonPolar substance dissolves in NonPolar

Substance– Polar substance will not dissolve in NonPolar

Substance

I. Determine what type of orbital the unpaired electron is in for each atom the bond is between

II. Draw a picture of each orbital before overlap

III. Overlap orbitals to show how bond forms

Valence bond theory – bonds are formed by sharing of e- from overlapping atomic orbitals.

Change in electron density as two hydrogen atoms approach each other.

10.3

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Hybridization – mixing of two or more atomic orbitals to form a new set of hybrid orbitals.

1. Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals.

2. Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process.

3. Covalent bonds are formed by:

a. Overlap of hybrid orbitals with atomic orbitals

b. Overlap of hybrid orbitals with other hybrid orbitals

10.4

Formation of sp Hybrid Orbitals

10.4

Formation of sp2 Hybrid Orbitals

10.4 10.4

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10.4

Determination of Atom Hybridization from Lewis Structures

Hybrid Orbital

# of Hybrid Orbitals

Total # HED

3-D Orbital Orientation

sp 2 2 Linear

sp2 3 3 TrigonalPlanar

sp3 4 4 Tetrahedral

10.4

Predict correctbond angle

10.5

sp2 Hybridized C

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Sigma bond (σ) – electron density between the 2 atomsPi bond (π) – electron density above and below plane of nuclei of the bonding atoms 10.5 10.5

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Sigma Bonding Animation

Double Bond in Ethylene

Pi Bonding Animation

Double Bond in Ethylene

spC

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10.5

Sigma (σ) and Pi Bonds (π)

Single bond 1 sigma bond

Double bond 1 sigma bond and 1 pi bond

Triple bond 1 sigma bond and 2 pi bonds

How many σ and π bonds are in the acetic acid(vinegar) molecule CH3COOH?

C

H

H

CH

O

O Hσ bonds = 6 + 1 = 7π bonds = 1

10.5

AB2 2 0 linear linear

Class

# of atomsbonded to

central atom

# lonepairs on


MolecularGeometry

VSEPR


trigonalplanar

10.1


AB5 5 0 trigonalbipyramidal

trigonalbipyramidal

AB6 6 0 octahedraloctahedral

10.1

12

10.1

Class

# of atomsbonded to

central atom

# lonepairs on


MolecularGeometry

VSEPR

10.1


trigonalbipyramidal

AB4E 4 1 trigonalbipyramidal

distorted tetrahedron

TA p298

Class

# of atomsbonded to

central atom

# lonepairs on


MolecularGeometry

VSEPR

10.1


trigonalbipyramidal



AB3E2 3 2 trigonalbipyramidal T-shaped

ClF

F

F

13

Class

# of atomsbonded to

central atom

# lonepairs on


MolecularGeometry

VSEPR-5 HED

10.1


trigonalbipyramidal



AB3E2 3 2 trigonalbipyramidal T-shaped

AB2E3 2 3 trigonalbipyramidal linear

I

I

I

Class

# of atomsbonded to

central atom

# lonepairs on


MolecularGeometry

VSEPR

10.1


AB5E 5 1 octahedral square pyramidal

Br

F F

FF

F

Class

# of atomsbonded to

central atom

# lonepairs on


MolecularGeometry

VSEPR – 6 HED

10.1


AB5E 5 1 octahedral square pyramidal

AB4E2 4 2 octahedral square planar

Xe

F F

FF

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Molecular Orbital Theory (MO)• Combine bonding atomic orbitals into

Molecular Orbitals

• 2 Atomic Orbitals 2 Molecular Orbitals

– ‘Wave-Function Properties’ of Orbitals

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Delocalized Electrons: Resonance

When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion.


• In reality, each of the four atoms in the nitrate ion has a p orbital.

• The p orbitals on all three oxygens overlap with the porbital on the central nitrogen.

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This means the π electrons are not localized between the nitrogen and one of the oxygens, but rather are delocalized throughout the ion.

Resonance

The organic molecule benzene has six σ bonds and a p orbital on each carbon atom.

Resonance

• In reality the π electrons in benzene are not localized, but delocalized.

• The even distribution of the π electrons in benzene makes the molecule unusually stable.

Molecular Orbital (MO) Theory

Though valence bond theory effectively conveys most observed properties of ions and molecules, there are some concepts better represented by molecular orbitals.

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• In MO theory, we invoke the wave nature of electrons.

• If waves interact constructively, the resulting orbital is lower in energy: a bonding molecular orbital.


If waves interact destructively, the resulting orbital is higher in energy: an antibonding molecular orbital.

MO Theory

• In H2 the two electrons go into the bonding molecular orbital.

• The bond order is one half the difference between the number of bonding and antibonding electrons.

MO Theory

For hydrogen, with two electrons in the bonding MO and none in the antibonding MO, the bond order is

12 (2 - 0) = 1

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MO Theory

• In the case of He2, the bond order would be

12 (2 - 2) = 0

• Therefore, He2does not exist.

MO Theory

• For atoms with both s and p orbitals, there are two types of interactions:– The s and the p orbitals that

face each other overlap in σfashion.

– The other two sets of porbitals overlap in πfashion.

MO Theory

• The resulting MO diagram looks like this.

• There are both σ and πbonding molecular orbitals and σ* and π* antibonding molecular orbitals.

MO Theory

• The smaller p-block elements in the second period have a sizeable interaction between the s and porbitals.

• This flips the order of the s and p molecular orbitals in these elements.

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Second-Row MO Diagrams

More electronegative-lowerin energy

No correspondingorbitals for these

lone-pair electrons

Oxygen is more electronegative

P orbitals present for bothAtoms form bonding-antibondingset of pi MO’s

Molecular Geometry and Hybridization of Atomic Orbitals

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