Hints for Assignment 1 CHEM 2P32


Review your first-year textbook on Lewis structures, VSEPR theory, resonance, formal charge, bond order, and hybridization

CHEM 190/191: Chang, 6th Edition, Chapters 9 and 10

CHEM 180/181: Kotz and Treichel, 4th Edition, Chapters 9 and 10
 
 

Example 1. Draw the Lewis structure of XeO2F2. Give the electron pair geometry at each atom, the hybridization of each atom, the formal charge on each atom, and the molecular shape for XeO2F2.

 
Atom
elect. pair geometry
hybridization
formal charge
Xe
trigonal bipyramid
sp3d
+2
Oa
tetrahedral
sp3
-1
Ob
tetrahedral
sp3
-1
Fa
tetrahedral
sp3
0
Fb
tetrahedral
sp3
0

Finally, the molecular shape of XeO2F2 is see-saw

Notes:

1. The formal charge of Xe is +2 because a neutral Xe atom would have 8 electrons around it, and after dividing the bonding electrons as shown above the Xe atom has only 6 electrons. Similarly, the formal charge of each O is -1 and the formal charge of each F is zero.

2. Get the hybridization from the electron pair geometry. Each electron pair geometry corresponds to a specific hybridization:
 

Electron pair geometry
Hybridization
linear
sp
triangular planar
sp2
tetrahedral
sp3
trigonal bipyramidal
dsp3 or sp3d
octahedral
d2sp3 or sp3d2
Example 2. Draw the resonance forms for the cyclic molecule S2N2 (sulfur alternates with nitrogen around the ring).
(a) How many sigma bonds are there? How many pi bonds?

(b) What is the average S-N bond order in this molecule?

(c) Arrange the following species in order of decreasing S-N bond strength: S2N2, NH2SH, SN+, SN-.

Answers. S2N2 has a total of 22 electrons. There are 4 resonance forms in which all atoms obey the octet rule.

(a) There are 4 sigma bonds and one pi bond in each resonance form. (Whenever there is a multiple bond, one bond is a sigma bond and the other is a pi bond.)

(b) The average S-N bond order is 5/4 = 1.25

(c) Bond strength depends on bond order. Therefore you need the Lewis structures of all four compounds.

SN+ has an S-N bond order of three (triple bond), SN- has an S-N bond order of two, and NH2SH has an S-N bond order of one (single bond).

Thus, the compounds in order of decreasing bond strength are SN+, SN-, S2N2, and NH2SH
 

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Last revised: 9 January 2001 by M. F. Richardson
© Brock University, 2001