Chemistry 20 - Chapter Molecular Shapes

VSEPR Theory VSEPR stands for Valence Shell Electron Pair Repulsion. VSEPR stands for Valence Shell Electron Pair Repulsion. The whole concept revolves around the idea that the electrons in a molecule repel each other and will try and get as far away from each other as possible. VSEPR explains a lot about molecular geometry and structure, BUT NOT EVERYTHING!! The whole concept revolves around the idea that the electrons in a molecule repel each other and will try and get as far away from each other as possible. VSEPR explains a lot about molecular geometry and structure, BUT NOT EVERYTHING!!

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

Electrons will come in several flavors: a) bonding pairs - this set of two electrons is involved in a bond, so we will write the two dots BETWEEN two atoms. This applies to single, double, and triple bonds. a) bonding pairs - this set of two electrons is involved in a bond, so we will write the two dots BETWEEN two atoms. This applies to single, double, and triple bonds. b) nonbonding pairs - this should be rather obvious. b) nonbonding pairs - this should be rather obvious. c) single electrons - in almost every case, this single electron will be nonbonding. c) single electrons - in almost every case, this single electron will be nonbonding.

VSEPR uses a set of letters to represent general formulas of compounds. These are: VSEPR uses a set of letters to represent general formulas of compounds. These are: a) A - this is the central atom of the molecule (or portion of a large molecule being focused on). a) A - this is the central atom of the molecule (or portion of a large molecule being focused on). b) X - this letter represents the ligands or atoms attached to the central atom. No distinction is made between atoms of different elements. For example, AX 4 can refer to CH 4 or to CCl 4. b) X - this letter represents the ligands or atoms attached to the central atom. No distinction is made between atoms of different elements. For example, AX 4 can refer to CH 4 or to CCl 4.

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

VSEPR shapes VSEPR shapes Linear Linear Angular Angular Trigonal Planar Trigonal Planar Trigonal Pyramidal Trigonal Pyramidal Tetrahedaral Tetrahedaral

AX 2 – Linear Example: BeH 2 Lewis Dot: H:Be:H Stereochemical Formula H-----Be-----H

AX 2 E – Angular Example: BrNO Lewis Dot Stereochemic al Formula

AX 3 - Trigonal planar Example: BF 3 Stereochemical Formula

AX 4 - Tetrahedral Example: CCl 4 Lewis Dot: Stereochemical Formula

AX 3 E - Trigonal pyramidal AX 3 E - Trigonal pyramidal Example: NH 3 Example: NH 3 Lewis Dot: Lewis Dot: Stereochemical Formula Stereochemical Formula

AX 2 E 2 - Angular Example: H 2 O Lewis Dot: Stereochemical Formula

AXE 3 - Linear Example: HCl Lewis Dot: H : Cl : Stereochemical Formula : H --- Cl..

Pages 91 –98 Practice Problems 1-8

Dipole Theory Polar Molecule: a molecule in which the negative charge is not evenly distributed around the molecule Polar Molecule: a molecule in which the negative charge is not evenly distributed around the molecule Non-polar molecule: a molecule with symmetrical distribution of the negative charge. Non-polar molecule: a molecule with symmetrical distribution of the negative charge.

Dipole Theory

Theoretical Prediction of Molecular Polarity To use molecular shape and bond polarity to predict the polarity of a molecule, complete these steps: To use molecular shape and bond polarity to predict the polarity of a molecule, complete these steps: Step 1: Draw a Lewis formula for the molecule. Step 1: Draw a Lewis formula for the molecule. Step 2: Use the number of electron pairs and VSEPR rules to determine the shape around each central atom. Step 2: Use the number of electron pairs and VSEPR rules to determine the shape around each central atom.

Theoretical Prediction of Molecular Polarity Step 3: Use electronegativities to determine the polarity of each bond. Step 3: Use electronegativities to determine the polarity of each bond. Step 4: Add the bond dipole vectors to determine whether the final result is zero (nonpolar molecule) or nonzero (polar molecule). Step 4: Add the bond dipole vectors to determine whether the final result is zero (nonpolar molecule) or nonzero (polar molecule). Sample 3.5 page 102 shows this well. Sample 3.5 page 102 shows this well.

Pages Read all. Take notes on Bolded terms. Copy Table 8 page 99 into notes Practice Problems 9-16 Section Questions 1-10