Homework Help: Chemistry

Posted by Mary on Monday, April 26, 2010 at 10:59pm.

Consider the Lewis structure for the major resonance form of PCS–. (Note that carbon is the central atom.) The structure shows:

a. A phosphorus-carbon triple bond and a carbon-sulfur single bond.
b. A phosphorus-carbon double bond and a carbon-sulfur double bond
c. A phosphorus-carbon single bond and a carbon-sulfur triple bond.
d. A phosphorus-carbon single bond and a carbon-sulfur double bond.
e. A phosphorus-carbon single bond and a carbon-sulfur single bond

I'm just mostly confused on doing lewis structures. I get the basic idea, but I'm confused on how you know to do single bonds/double bonds. Or how to know when a bond is polar. Also, when a molecule is polar.

Chemistry - DrBob222, Monday, April 26, 2010 at 11:17pm
We can't draw structures on this board but perhaps I can help clear up some of your confusion.
PCS. Count the electrons.
P = 1 x 5 = 5
C - 1 x 4 = 4
S = 1 x 6 = 6
1 neg charge = 1
Total = 16.

There are 3 atoms, they each want 8 so 3 x 8 = 24 and we have 16. That is 24-16 = 8 short and since we usually pair electrons to make a bond that is 8/2 = 4 bonds.

So I would put P::C::S which takes care of 8 electrons and we need to add 8 more.I would add 4 more to P and 4 to S. That gives 8 around C, 8 for P and 8 for S. Then I count the formal charge. On P, FC is -1 (and the ion has a -1 charge), C is zero and S is zero. Molecules generally don't like formal charges (at least formal charges that are +/- 2 or +/- 3. So I would count that as the major structure. You can draw others by shifting the electrons around.
For polarity. We need the shape, I count the regions of high electron density. I see two for C and that makes it linear. I see P has an electronegativity of 2.19 and S is 2.58 and since these are exactly opposite each other, there is a slight polarity to the molecule (but not all that much).

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