Chem 2045	Discussion session	Return
Work sheet 14		Answers are in red

1) Use the following template to determine if each of the following molecules is polar or non-polar: CH₂O, CH₄, CH₃OH,  PCl₆^-, XeF₄

CH2O	CH4	CH3OH	PCl6-	XeF4
Lewis dot structure:	Lewis dot structure:	Lewis dot structure:	Lewis dot structure:	Lewis dot structure: I have left the lone pairs off the F's for clarity.
Number of electron domains: 3	Number of electron domains: 4	Number of electron domains: 4, 4	Number of electron domains: 6	Number of electron domains: 6
Electron domain geometry: Trigonal planar	Electron domain geometry: Tetrahedral	Electron domain geometry: Tetrahedral, Tetrahedral	Electron domain geometry: Octahedral	Electron domain geometry: Octahedral
Molecular geometry: Trigonal planar	Molecular geometry: Tetrahedral	Molecular geometry: Tetrahedral, Bent	Molecular geometry: Octahedral	Molecular geometry: Square planar The lone pairs should be across from each other
Polar: Yes	Polar: No	Polar: Yes	Polar: No	Polar: No

2) The molecules CH₄, NH₃ and H₂O all have the same electron domain geometry, but they each have a different molecular geometry and different bond angles. Explain this observation using VSEPR theory.

All of the molecules have 4 electon domains around the central atom so they all have a "tetrahedral" electron domain geometry. However, while the CH₄ has no lone pairs (and a "tetrahedral" molecular geometry) the NH₃ has one lone pair (so it has a "trigonal pyramidal" molecular geometry) and H₂O has two lone pairs (giving it a "bent" molecular geometry). The increasing number of lone pairs results in a decrease in the bond angles in the molecule (as shown on page 310 of your text).