1. Molecular shapes

2. Learning objectives
Apply VSEPR to predict electronic geometry and shapes of simple molecules

3. Valence shell electron pair repulsion
Lewis dot structure provides 2D sketch of the distribution of the valence electrons among bonds between atoms and lone pairs; it provides no information about molecular shape
First approach to this problem is to consider repulsion between groups of electrons (charge clouds)

4. Electron groups (clouds) minimize potential energy
Valence shell electron pair repulsion (VSEPR)
Identify all groups of charge: non-bonding pairs or bonds (multiples count as one)
Bonded atoms – single, double or triple count as 1
Lone pairs count as 1
Distribute them about central atom to minimize potential energy (maximum separation)

5. Total number of groups dictates electronic geometry
Octet rule:
Two – linear
Three – trigonal planar
Four – tetrahedral

6. Three groups: trigonal planar
Two possibilities for central atoms with complete octets:
Trigonal planar (H2CO)
Bent (SO2)
BCl3 provides example of trigonal planar with three single bonds
B is satisfied with 6 electrons

7. Four groups: tetrahedral
Three possibilities:
No lone pairs (CH4) - tetrahedral
One lone pair (NH3) – trigonal pyramid
Two lone pairs (H2O) – bent
Note:
H-N-H angle 107°
H-O-H angle 104.5°
Tetrahedral angle 109.5°

8. Representations of the tetrahedron

9. Groups of charge
Lone electron pairs
Electronic geometry
Molecular shape 2
Linear 3
Trigonal planar 1
Bent 4
Tetrahedral
Trigonal pyramid

10. Two bonds Equal bonds oppose (linear) Unequal bonds oppose (linear)
Nonpolar (CO2)
Unequal bonds oppose (linear)
Polar (HCN)
Equal bonds do not oppose (bent)
Polar (H2O)

11. Three bonds Equal bonds oppose in trigonal planar arrangement
Nonpolar
Unequal bonds in trigonal planar arrangement
Polar