Molecular Geometry (VSEPR)

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Molecular Geometry(VSEPR)

Molecular Structure

YouTube

Video on VSPER Theory

Learning Objectives

TLW predict molecular structure for molecules using Valence Shell Electron Pair Repulsion (VSEPR) Theory (TEKS 7.E)

A. VSEPR Theory

1. Valence Shell Electron Pair Repulsion Theory gives us a three-dimensional picture of atomic bonding that the Electron Dot Structure does not.

2. Electron pairs orient themselves in order to minimize repulsive forces.

VSEPR:

3. Predicts three dimensional geometry of molecules.

4. The name tells you the theory:

5. Valence shell - outside electrons.

6. Electron Pair repulsion - electron pairs try to get as far away as possible.

7. Can determine the angles of bonds.

A. VSEPR Theory

Types of e- Pairs

1. Bonding pairs - form bonds

2. Lone pairs - nonbonding e-

Lone pairs repel

more strongly than

bonding pairs!!!

A. VSEPR Theory Lone pairs reduce the bond angle

between atoms.

Bond Angle

1. Draw the Lewis Diagram.

2. Tally up e- pairs on central atom.a. double/triple bonds = ONE pair

3. Shape is determined by the # of bonding pairs and lone pairs.

Know the 3 most common shapes & their bond angles!

B. Determining Molecular Shape

C. Common Molecular Shapes 1

2 total

2 bond

0 lone

LINEAR180°BeH2

CO2

O C O2 total

2 bond

0 loneLINEAR

180°

Examples

C. Common Molecular Shapes 2

3 total

2 bond

1 lone

BENT

<120°

SO2

3 total

3 bond

0 lone

TRIGONAL PLANAR

120°

BF3

C. Common Molecular Shapes 3

4 total

4 bond

0 lone

TETRAHEDRAL

109.5°

CH4

C. Common Molecular Shapes 4

4 total

3 bond

1 lone

TRIGONAL PYRAMIDAL

107°

NH3

C. Common Molecular Shapes 5

PF3

4 total

3 bond

1 lone

TRIGONAL PYRAMIDAL

107°

F P FF

Examples

4 total

2 bond

2 lone

BENT

104.5°

H2O

C. Common Molecular Shapes 6

5 total

5 bond

0 lone

TRIGONAL BIPYRAMIDAL

120°/90°

PCl5

C. Common Molecular Shapes 7

6 total

6 bond

0 lone

OCTAHEDRAL

90°

SF6

C. Common Molecular Shapes 8

Examples linear: BeH2, CO2, MgF2, I3 bent (angular): SO2, H2O, H2S, SF2 square planar: XeF4, IF4

-

trigonal planar: SO3, BF3 square pyramidal: IF5, BrF5 trigonal pyramidal: NH3, PF3, AsCl3 trigonal bipyramidal: PF5, PCl5, AsF5 tetrahedral: CH4, CF4, SO4

2- octahedral: SF6, PF6

-, SiF62-

seesaw: SF4 T-shaped: ClF3

D. Orbital Hybridization

VSEPR Theory works well when accounting for molecular shapes, but doesn’t help describing the types of bonds formed.

In hybridization, several atomic orbitals mix to form the same total number of equivalent hybrid orbitals

YouTube video

D. Orbital Hybridization

For Example ~ Methane (CH4)

C = 1s22s22p2

H = 1s1 (and there are four H atoms)

C re-configures its one 2s and three 2p orbitals into four sp3 orbitals, which overlap the 1s orbitals of the 4 hydrogen atoms

Group Practice

Molecular Geometry Construction Game Revisited

E. One Other Note on Bonding Electronegativity determines bonding

which contributes to the bond angle Greater than 1.7 ionic bonds 0.3 – 1.7 polar covalent bonds 0 – 0.3 covalent

See handout for trends through periodic table

Electronegativities• Greater than 1.7 ionic bonds• 0.3 – 1.7 polar covalent bonds• 0 – 0.3 covalent

Independent Practice

Building 3-D Examples of Molecular Geometry VSEPR Marshmellow Lab

VSEPR Exercise – 2