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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
