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).
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.
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