Chemistry 20 - Chapter 3

3.3 Molecular Shapes

VSEPR Theory VSEPR stands for Valence Shell VSEPR stands for Valence Shell

Electron Pair Repulsion. Electron Pair Repulsion. The whole concept revolves The whole concept revolves

around the idea that the around the idea that the electrons in a molecule repel electrons in a molecule repel each other and will try and get each other and will try and get as far away from each other as as far away from each other as possible. VSEPR explains a lot possible. VSEPR explains a lot about molecular geometry and about molecular geometry and structure, BUT NOT structure, BUT NOT EVERYTHING!!EVERYTHING!!

VSEPR Theory The electrons (both in The electrons (both in

pairs and singles as you pairs and singles as you will see) are "attached" to will see) are "attached" to a central atom in the a central atom in the molecule and can "pivot" molecule and can "pivot" freely on the atom's freely on the atom's surface to move away from surface to move away from the other electrons.the other electrons.

Electrons will come in several flavors: a) bonding pairs - this set of a) bonding pairs - this set of

two electrons is involved in a two electrons is involved in a bond, so we will write the bond, so we will write the two dots BETWEEN two two dots BETWEEN two atoms. This applies to single, atoms. This applies to single, double, and triple bonds.double, and triple bonds.

b) nonbonding pairs - this b) nonbonding pairs - this should be rather obvious.should be rather obvious.

c) single electrons - in c) single electrons - in almost every case, this almost every case, this single electron will be single electron will be nonbonding. nonbonding.

VSEPR uses a set of letters to VSEPR uses a set of letters to represent general formulas of represent general formulas of compounds. These are:compounds. These are:

a) A - this is the central atom of a) A - this is the central atom of the molecule (or portion of a large the molecule (or portion of a large molecule being focused on).molecule being focused on).

b) X - this letter represents the b) X - this letter represents the ligands or atoms attached to the ligands or atoms attached to the central atom. No distinction is central atom. No distinction is made between atoms of different made between atoms of different elements. For example, AXelements. For example, AX44 can can refer to CHrefer to CH44 or to CCl or to CCl44..

c) E - this stands for c) E - this stands for nonbonding electron pairs.nonbonding electron pairs.d) e - this stands for lone d) e - this stands for lone nonbonding electrons.nonbonding electrons.

VSEPR shapesVSEPR shapesLinearLinearAngularAngularTrigonal PlanarTrigonal PlanarTrigonal PyramidalTrigonal PyramidalTetrahedaralTetrahedaral

AX2 – Linear

Example: BeH2

Lewis Dot: H:Be:H Stereochemical Formula

H-----Be-----H

AX2E – Angular

Example: BrNO

Lewis Dot Stereochemical Formula

AX3 - Trigonal planar

Example: BF3 Stereochemical Formula

AX4 - Tetrahedral

Example: CCl4

Lewis Dot: Stereochemical Formula

AXAX33E - Trigonal E - Trigonal pyramidal pyramidal

Example: Example: NHNH33

Lewis Dot: Lewis Dot: Stereochemical Stereochemical FormulaFormula

AX2E2 - Angular

Example: H2O

Lewis Dot: Stereochemical Formula

AXE3 - Linear

Example: HCl

Lewis Dot: H:Cl:Stereochemical Formula: H---Cl

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Pages 91 –98Pages 91 –98

Practice Practice Problems 1-8Problems 1-8

Dipole Theory Polar Molecule: a molecule Polar Molecule: a molecule

in which the negative charge in which the negative charge is not evenly distributed is not evenly distributed around the moleculearound the molecule

Non-polar molecule: a Non-polar molecule: a molecule with symmetrical molecule with symmetrical distribution of the negative distribution of the negative charge.charge.

Dipole Theory

Dipole Theory

Theoretical Prediction of Molecular Theoretical Prediction of Molecular PolarityPolarity To use molecular shape and To use molecular shape and

bond polarity to predict the bond polarity to predict the polarity of a molecule, polarity of a molecule, complete these steps:complete these steps:

Step 1: Draw a Lewis formula Step 1: Draw a Lewis formula for the molecule.for the molecule.

Step 2: Use the number of Step 2: Use the number of electron pairs and VSEPR electron pairs and VSEPR rules to determine the shape rules to determine the shape around each central atom.around each central atom.

Theoretical Prediction of Molecular Theoretical Prediction of Molecular PolarityPolarity Step 3: Use electronegativities Step 3: Use electronegativities

to determine the polarity of to determine the polarity of each bond.each bond.

Step 4: Add the bond dipole Step 4: Add the bond dipole vectors to determine whether vectors to determine whether the final result is zero the final result is zero (nonpolar molecule) or nonzero (nonpolar molecule) or nonzero (polar molecule).(polar molecule).

Sample 3.5 page 102 shows Sample 3.5 page 102 shows this well.this well.

Pages 98-104Pages 98-104Read all.Read all.Take notes on Bolded Take notes on Bolded terms.terms.

Copy Table 8 page 99 into Copy Table 8 page 99 into notesnotes

Practice Problems 9-16Practice Problems 9-16Section Questions 1-10Section Questions 1-10