Chapter 9: Molecular Geometry and Bonding Theories Chem 121A Dr Fetto

VSEPR Model

Valence shell electron pair repulsion (VSEPR) Model utilizes elements domains to determine 3D structure.

The difference between the 2D Lewis vs 3D molecule.

Electron Domains

Can use Lewis structures to determine the various domains in our molecule.

Bonding domains

Generated in areas in which there is a chemical bond

• multiple nods count as a single bonding domain

O\\O\O

Nonbonding domains

Are generated by lone pairs on the central atom

• one lone pair is two electrons

Electron domain geometries

ED: 2 Is linear with 180 degree bond angle.

3 ed is trigonal planar with 120 degree bond angles.

4 ed is tetrahedral with 109.5 degree bond angles.

5 ed is triognal bipyramidal with 120 degree and 90 degree bond angles. It has equatorial and axial bond angles.

6 ed is octahedral with 90 degree bond angles.

2 electron domain

Linear

180 degrees Bond angles

3 electron domains

Trigonal planar

120 degrees bond angles

4 electron domains

Tetrahedral

109.5 bond angles

Molecular geometry

Electron domain geometry is simple and doesn’t take the difference between bonding and nonbonding domains.

Nonbonding are larger than bonding domains.

Electrons repel each other, making them spread apart. This distorts geometry

Molecular geometry accounts for the difference in size in domains and the effect on the shape.

Bent

2 bonding domains

1 nonbonding domain

3 total electron domain

<120 degree

4 electron domains

3 bonding domains

1 nonbonding domain

4 total electron domains

Trigonal pyramidal

<109.5

4 electron domains again

2 bonding domains

2 nonbonding domains

Bent

«109.5

5 electron domains a

Remove from equatorial side first.

4 bonding domains

1 Nonbonding domains

Seesaw

Axial: <90

Equatorial: <120

5 electron domain b

3 bonding domains

2 nonbonding domains

T shaped

Axial: <90

Equatorial: no more equatorial

5 domains c

2 bonding domains

3 nonbonding domains

Linear

Axial: 180

No more equatorial

6 domains

5 bonding domains

1 nonbonding domains

Square pyramidal

Axial: <90

Equatorial: 90

6 domains a

4 domains

2 nonbonding domains

Square planar

Axial: no axial

Equatorial: 90

Bond polarity

Measure of how equal or unequally the electrons are shared in a covalent bond

Nonpolar bond

When 2 identical atoms bond the electrons are shared and distributed equally

Polar bond

When two different atoms bond and there is an uneven distribution of electrons

Electronegativity

Bond polarity is determined by electronegativity. Electronegativity is the ability of an atom in a molecule to attract electrons towards itself.

Nonpolar: 0

Polar: 0 < x < 2

Ionic x ≥ 2

Dipole moments

The uneven distributions of electrons in a polar bond. The uneven distribution will generate a dipole moment (μ)

Dipole moments are denoted by a crossed arrow with the arrow pointing towards the most electronegative atom.

How to Pol your arity

Imagine all atoms are playing tug-o-war with the central atom

• Between atoms: the most electronegative atom will “pull” the

most

• Lone pairs will “pull” on the central atom stronger than atoms

• If there is equal “pull” in 3D space: nonpolar

• If there is unequal “pull” in 3D space: po

Valence Bond Theory

Bonding electron pairs occur when atomic orbitals “overlap”

Bond length & strength

In valence bond theory

As orbital overlap increases, bond strength increases.

When atoms get too close, electrons repel each other.

Hybrid orbitals

Orbitals of different atoms will mix to form hybrid orbitals which dictate geometry.

Hybrid orbitals form to generate a space in which an electron from each outer atoms can pair with a hybrid orbital form the central atom.

Different combinations of s and p orbitals will form different hybrids.

Multiple Bonds

When we have overlap along a consistent linear axis, we create a sigma bond

When we have parallel overlap of p orbitals we create a pi bond

The type of overland that occurs will determine if we have a single, double, or triple bond.

Determine sigma and pi bonds

Single: 1 pi bond

Double: 1 sigma + 1 pi bond

Triple: 1 sigma + 2 pi bonds