1. Resonance Structures
Some molecules are not well described by Lewis Structures.
Typically, structures with multiple bonds can have similar structures with the multiple bonds between different pairs of atoms

2. Resonance Structures
Example: experimentally, ozone has two identical bonds whereas the Lewis Structure requires one single (longer) and one double bond (shorter).

3. Valence Shell Electron Pair Repulsion Theory (VSEPR)
Electron pairs repel as much as possible in 3-dimensional space.
Theory used to predict shapes of molecules.

5. To determine the shape of a molecule, we distinguish between lone pairs (or non-bonding pairs, those not in a bond) of electrons and bonding pairs (those found between two atoms).
The electrons adopt an arrangement in space to minimize e--e- repulsion.

7. Examples – Draw the Lewis structures, and then determine the orbital geometry of each:
H2S
CO2
PCl3
CH4
SO2

8. Molecules with Expanded Valence Shells
Atoms that have expanded octets have AB5 (trigonal bipyramidal) or AB6 (octahedral) electron pair geometries.
For trigonal bipyramidal structures there is a plane containing three electron pairs. The fourth and fifth electron pairs are located above and below this plane.
For octahedral structures, there is a plane containing four electron pairs. Similarly, the fifth and sixth electron pairs are located above and below this plane.

11. Molecules with Expanded Valence Shells
To minimize e--e- repulsion, lone pairs are always placed in equatorial positions.

12. Examples – Determine the Shape of each:
PF5
XeF4
SF6
SCl4

13. Examples – Determine the Shape of each:
PF5 trigonal bipyramid 90°, 120°
XeF4 square planar 90°
SF6 octahedral 90°
SCl4 see-saw 90°, 120°