Shapes of Molecules

1
Shapes of Molecules

• Determined by number of valence electrons of the
  central atom
• 3-D shape a result of bonded pairs and lone pairs
  of electrons
• Use VSEPR theory (valence-shell-electron-pair
  repulsion

2
What is VSEPR Theory

• Valence Shell Electron Pair Repulsion Theory-
  This just means that electrons will adjust
  themselves as far away from each other as
  possible.
• Non-bonded electrons (unshared) have more of a
  repulsion.

3
Using the VSEPR Model

• 1. Identify the central atom
• NEVER HYDROGEN This is a terminal atom.
• Usually it is the first element in the compound.
  
• 2. Draw the electron-dot structure
• 3. Count the total number of electron pairs
  around central atom
• 4. Predict the shape of the molecule using the
  bonding atoms

4
Shapes of Molecules

• Number of bonded atoms 2
• Name of shape linear
• There are NO unshared pairs of electrons on the
  central atom

5
BeCl2

• Be 2 valence electrons
• .. ..
• Cl Be Cl
• .. ..
• .. ..
• Cl Be Cl linear
• .. ..

6
More Electron Pairs

• Atoms Bonded Pairs Shape
• 3 3 trigonal planar

7
Electron Shape with 3 Pairs

8
BF3

• B has 3 valence electrons
• F trigonal
• planar
• B
• F F
• No unshared pairs on the central atom

9
More Electron Pairs

• Electron Bonded Name of
• Pairs Atoms Shape
• 4 4 tetrahedral
• 4 3 pyramidal
• 4 2 bent

10
Tetrahedral CH4

• 4 electron Pairs, 4 bonded atoms,
• NO UNSHAIRED
  PAIRS

11
NH3 Pyramidal

• N has 4 electron pairs, 3 bonded H atoms
• Shape is called pyramidal
• ONE UNSHARED PAIR
• ..
• H N H
  N
• .. H H
• H H

..
12
H2O Bent or Angular

• O has 4 electron pairs, 2 bonded H atoms
• Shape is called bent
• 2 UNSHARED PAIRS
• ..
• O H
  O
• .. H
• H H

..
..
13
Learning Check

• Determine the molecular geometry of each of the
  following
• A. CCl4
• 1) tetrahedral 2) pyramidal 3)Bent
• B. SO3
• 1) trigonal planar 2) pyramidal 3) Bent
• C. PCl3
• 1) trigonal planar 2) pyramidal 3) Bent

14
Solution

• Determine the molecular geometry of each of the
  following
• A. CCl4
• 1) tetrahedral
• B. SO3
• 1) trigonal planar
• C. PCl3
• 2) pyramidal