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
Using the VSEPR Model

• 1. Draw the electron-dot structure
• Identify the central atom
• Count the total number of electron pairs around
  central atom
• Predict the electron shape
• Predict the shape of the molecule using the
  bonding atoms

3
Shapes of Molecules

• Number of electron pairs 2
• ( negative charge clouds)
• Number of bonded atoms 2
• Angle 180
• Name of shape linear

4
BeCl2

• Be 2 valence electrons
• .. ..
• Cl Be Cl
• .. ..
• .. ..
• Cl Be Cl linear, 180
• .. ..

5
More Electron Pairs

• Electron Angle Bonded Name of
• Pairs Atoms Shape
• 3 120 3 trigonal planar
• 3 120 2 angular

6
Electron Shape with 3 Pairs

7
BF3

• B has 3 valence electrons 120
• F trigonal
• planar
• B
• F F

8
SO2

• S has 2 bonded atoms , 1 lone pair (electron
  cloud)
• 120, angular
• .. .. ..
• O SO S
• ..
• O O

9
More Electron Pairs

• Electron Angle Bonded Name of
• Pairs Atoms Shape
• 4 109 4 tetrahedral
• 4 109 3 pyramidal
• 4 109 2 bent, angular

10
Tetrahedral CH4

• 4 electron Pairs, 4 bonded atoms, 109
• H
  H
• C
  H C H
• H
• H H
  H

11
NH3 Pyramidal

• N has 4 electron pairs, 3 bonded H atoms
• Shape is called pyramidal
• ..
• 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 or angular, 109
• ..
• 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) angular
  
• B. SO3
• 1) trigonal planar 2) pyramidal 3) angular
• C. PCl3
• 1) trigonal planar 2) pyramidal 3)
  angular

14
Solution

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