Questions about molecular forces and shapes

[sanjuro]

Hi
I wonder if you could help me with the following multiple choice questions:

[1] Which of the following does not have a dipole moment

a) H20 b)NH3 c)CO2 D)SF4 E)BRCL5

Are we just suppose to know that the answer is CO2 or is
there some simple way of working it out?


[2] The Lewis structure of PCl3 indicates four regions of high electron density around the phosphorus atom: one lone pair and three single bonds. What is the shape of the PCl3 molecule?

[a] Tetrahedral (b)trigonal pyramidal c)trigonal planar e) T-shape

How do we know that the answer is a trigonal pyramidal shape?

3] Similar sort of question - what is the molecular structure of IF6+
(a) octahedral (b) trigonal bypyramidal (c) square pyramidal (d) trigonal planer

In this case the number 6 denotes that it is an octahedral but often the answer is not so straightforward when non bonding pairs are concerned. How do we know if there are non bonding pairs?

 

[GCT]

For question 1:

Use the valence bond theory to deduce the shapes of the molecules and from this you can deduce if it has a dipole moment. Carbon dioxide has 16 valence electrons; 4 from carbon, 12 from both oxygens. Now draw one possible structure; with the carbon in the middle.

-Connect the atoms through a single bond.

-Make sure that both carbon and oxygen has a full octet by distributing the lone pairs. As you can see, we do not have enough electrons. Through single bonds we will need more than 16 electrons.

-So make another proposal. Connect the oxygen to carbon through double bonds. Make sure that each atom has eight electrons around it. This structure satisfies the valence bond theory. Notice that this structure does not have any lone pairs. Thus the central carbon is sp hybridized; it is a pure linear molecule.

For question 2 study ammonia; look into a general chemistry textbook. Trigonal pyramidal is more specific in indicating an sp3 hybridized central atom with a lone pair.

For question 3 look into what I said for the question number 1.

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