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so formal charges allow us to figure out

which is going to be the best structure

for a chemical compound and by best I

mean the one that's most likely to exist

in the real world so let's start with a

simple one let's look at ch4 methane to

calculate formal charges we do that for

each atom so let's take a look at the

carbon first carbon on the periodic

table is in group 4 or 14 4 valence

electrons minus nonbonding these are the

ones that aren't formed in chemical

bonds all of these electrons for carbon

they're bonded so we don't have any

nonbonding and then bonding electrons we

have all of these right here so we have

a total of 8 bonding valence electrons

divide that by 2 4 minus 0 minus 4 is 0

so we can write a formal charge for

carbon and put it right here in the

middle as 0 so now let's try the formal

charge for hydrogen since all the

hydrogen's are the same they're

symmetrical we only need to do one so

valence electrons hydrogen's in Group

one so one valence electron nonbonding

these guys are all bonded so 0 and then

the 2 bonding valence electrons divided

by 2 1 minus 1 equals 0 so the formal

charge for hydrogen is 0 and that's the

same for all the hydrogen's since it's

symmetrical so that's methane so we've

calculated the formal charges for

methane not too bad if you can remember

the formula so often you'll have more

than one possible lewis structure and

this happens quite frequently when you

have something like sulfur in period 3

Row 3 of the periodic table or

phosphorus in the same period they can

have more than eight valence electrons

and that opens up some possibilities so

you want to check the formal charges

especially when you see sulfur or

phosphorous so let's try it we'll do

this

oxygen first and oxygen is in group six

so it has six valence electrons

nonbonding that's these ones on the

outside

sign has six of those and then bonding

we have a bond right there 2 divided by

2 so 6 minus 6 minus 1 is minus 1 formal

charge on that oxygen is minus 1 with a

sulfur right here we have group 6

nonbonding that's these here and then

bonding these and these are forming

bonds so we have 6 over 2 & 6 minus 2

minus 3 equals a positive 1 so this is

going to have a formal charge of plus 1

for the last oxygen we have six valence

electrons nonbonding these here and here

so 4 and then bonding 4 of those divided

by 2 6 minus 4 minus 2 is 0 that has a

formal charge of 0 let's try this

molecule over here so we're going to

this oxygen we have 6 minus 4 and then

these are our bonding so 4 over 2 that

equals 0 formal charge of 0 for the

sulfur we have 6 minus nonbonding minus

the bonding

so we have eight over to there and that

equals 6 minus 2 minus 4 we have 0 again

formal charge of 0 finally the last

oxygen here we have 6 minus 4 nonbonding

4 bonding over to 6 minus 4 minus 2 a 0

again formal charge of 0 so when we look

at these structures we want to choose

the one that has the formal charges

close to 0 and in this case that's right

here this is the structure that has the

formal charges closest to 0 and

therefore that's going to make it the

most appropriate or the best lewis

structure the one that will likely

define in the real world

you've got your equation for figuring

out formal charges you can calculate

them for each atom and then compare

different structures it's a little bit

confusing at first but with practice it

gets pretty easy this is dr. B and

thanks for watching