If anyone could explain this to me I would REALLY appreciate it.
For example: CS2 ( little two).
To calculate the Valence electrons we do 4 + 12= 16 – 4= 12
I know that C is the group that carbon is in on the periodic table counting across. Do we get the 12 from taking where Sulfer is ( 6?) and multiple it by 2? Also, why are we subtracting 4?
In the lewis dot structure how do we know how to divide up the dots and how do we know if it is bent, linear, tetrahedral…etc.
2 Answers

to calculate the number of valence electrons you look at how much each atom has:
So for the S atom, it has 6 valence electrons each and there are 2 atoms.
now for the C atom, it has 4 valence electrons and there is only 1.
So the total number of valence electrons = S+S+C = 6+6+4 = 16 total valence electrons. I think what you did when you subtracted 4 was you were finding the total number of valence electrons for ONLY sulfur so you subtracted the valence electrons that carbon contributed (4 valence electrons). but that’s not the total number of valence electrons for the molecule. 16 is the right answer.
ok as for your lewis dot structure. what you need to start out with is to see how the molecules are placed. so you have 1 carbon and 2 sulfurs so the odds are that carbon is going to be your central atom.
SCS
now you put all your electrons on and see what you have. each bond makes 2 electrons. but remember you can only work with 16 electrons because that’s the number of valence electrons that can be used for bonding. so you have 2 bonds so far which makes 4 electrons. 2+2 = 4…. SO 164 = 12. now you only have 12 electrons to work with. and you know that everything should have octets so just stick a bunch of dots around all the atoms so that each atom has 8 electrons INCLUDING the bonding electrons.
:SCS: (plus 4 more dots on each atom…it’s hard to draw on the computer)
now when you count the electrons you will notice that you have too many electrons (20 electrons) so what you need to do is make some more bonds. here’s a pattern in chemistry.. carbon likes to have 4 bonds always so just remember that. so if you make carbon have 4 bonds then you should have a structure like this:
S=C=S
now start counting electrons. so far you have 8 bonding electrons. so from your original 16 valence electrons — 168 = 8 electrons left over.
so now that carbon has an octet (8 bonded electrons) stick the remaining 8 electrons equally on the two sulfurs.
:S=C=S: (plus two more on top of each sulfur.)
now count your electrons and you’ll see that you have 4 unpaired electrons on each sulfur which makes 8 plus 8 bonded electrons and that makes your 16 valence electrons. so that’s your lewis structure.
now to know if it’s bent or whatnot you know that a double bond is short and strong. so it’s going to to be too strained to be bent. so it’s going to be linear because its a carbon bonded to two things so it cant be tetrahedral or trigonal planar or any of those sorts. it has to do with the hybridization of the molecular orbitals but i think that’s a little too advanced for what you’re learning. so it’s linear. here’s an oversimplification of what i mean though: http://www.elmhurst.edu/~chm/vchembook/images/207e…
hope this helps! goodluck!

Cs2 Lewis Structure