let's talk about drawing Lewis structures so most textbooks are a little bit convoluted in that
they try to oversimplify the rules and you end up running into situations where that you where you
can't actually draw a lewis structure or they try to be so precise and their language isn't clear
and you end up going down rabbit trails so I've tried to distill these rules for drawing Lewis
structures as simply as I can and we're in again the best way to do Lewis structures eventually
is just to do examples until you get the hang of it so we've got four of those at the end of this
video to help so first rule we need to count the valence electrons in all of our atoms and so we
know Group 1 group 2 group 3 4 5 6 7 8 the group number the main group elements just tells us what
the valence electrons are the D block and the F block only have two valence electrons the ones
that arise from the main group okay from the outer shell because remember when you're counting the
4s is here row four but this is the 3d row okay so it's inner shell electrons not valence okay
that's an aside we won't be running into many transition metals in this discussion of Lewis
structures so just the main group elements are what we are primarily concerned with and they're
numbers of electrons that will get here okay so the next rule and again correcting for polyatomic
ions we'll talk about with an example we want to connect the central atom to outer atoms with
a single bond and the central atom is going to be the least electronegative so if we know
that fluorine is the most electronegative element and in electronegativity increases up and to the
right so the farthest away from chloro flourine is going to be our central atom also if there's
only one type of atom that's going to be the central atom or if it's carbon it in an organic
compound Carbon loves to be the central atom at all times after we connected our outer atoms to
the central atom of the single bond we're going to fill the valence's of our outer atoms noting
that H cannot get lone pairs and that the others will get a full octet we want to put
any leftover electrons on the central atom and if the central atom does not have an octet after
this step we'll use multi bonds to do this and to do that we're converting a lone pair into a multi
bond ok minimize for formal charges if possible during this step then finally we'll add the formal
charges to our Lewis structure so a lot of rules but it's best to just put those into practice okay
so let's do our example the first one ccl4 so as you learned you learn dot structures
for a reason because they loosely get you to a lewis structure if we take four chlorines which
have a dot structure with 7 valence 1 unpaired a chlorine R excuse me a carbon with four unpaired
if you had four chlorines you could envision pairing up the electrons in dot structures
to get our full octet on the carbon and share the electrons so we're beyond this now we are now at
the point to where we can do Lewis structures and get rid of all the dots okay and so that's what
we're going to do how let's start with our rules count the number of valence electrons carbon and
four chlorines give us four from column four four times seven 28 plus 4= 32 total electrons in this
molecule connect to the central atom Carbon loves to be the central atom of it it also is the most
electropositive of the two so we connect with a single bond then we fill the valenc's of the
outer atoms two four six the bond counts this two so it's now got eight octet two four six
eight already have two two four six eight all of these are the same now we look eight times
we have eight eight eight and eight so eight times four is thirty-two we're out of electrons and so
we're done with this Lewis structure okay formal charge if we look at the formula for formal charge
it's the number of valence minus the number of bonds minus the number of dots from the from
the lone pairs on the element so if we look at chlorine chlorine is normally seven it's got one
bond and six dots formal charge of zero if you do the same calculation for carbon you'll have
four valence minus four bonds zero dots it's still zero formal charge so we're done with
this Lewis structure okay let's look at the next example this is per chlorate polyatomic ion per
chlorate let's count the electrons rule number one one chlorine four oxygens correct for ionic charge
negative one okay so we've got a correct for the number of electrons that have been added based on
the charge correction so I always show that as an electron correction okay and so 7 4 times 6
for oxygen and 1 extra electron for the negative charge there'd been negative 1 there if it were a
positive charge so 24 plus 25 plus 1 is 25 plus 7 is 32 electrons central atom least electronegative
is chlorine connect to the outer atoms 4 oxygens fill the valence's of the outer and
when we count we get 32 so we're done now we have to put formal charge in each oxygen if you do the
calculation has a negative 1 formal charge 6 minus 1 bond is 5 minus 6 dots is negative 1
okay so negative there chlorine likes to have 7 valence so 7 minus 4 bonds is plus 3 plus 3 minus
4 is negative 1 so the total charge there there are four - 1's and a positive 3 that's a total
of negative 1 so we have balanced out here and we are good I do not do expanded octet okay if
your book tries to get you to do so on a problem you're free to do that but I will accept either
with expanded octet or without I don't think it's necessary because I think it's more confusing
than we get out of it we can still get all the information we need without the controversial
expanded octet even the textbooks can't agree on whether it actually exists ok so we're gonna
stay with this structure and we'll see if what the molecular structure it's a tetrahedral structure
just like expanded octet would predict ok all right so we're done there let's try two more
how about socl2 so count them up no charge sulfur six oxygen is six chlorine is seven so 14 plus six
there's twenty plus six is twenty six electrons and the least electronegative is sulfur connect
to the outer atoms anywhere we want to place the oxygen versus a chlorine is fine but we have to
fill the octets of the outer and so now we are at 24 electrons okay and so 24 electrons 8 8 8
okay we need two more they go on the central atom and that completes the octet of sulfur 2
4 6 8 now so if we do formal charge we have a negative 1 here a positive 1 on the sulfur and
we are done with our Lewis structure what about formaldehyde okay so we've got C two H's and an
oxygen carbon has four valence hydrogen has one in Group one oxygen has 6 so 6 plus 2 is 8 plus
4 is 12 electrons central atom carbon trumps everything connect one single bond to every
outer atom fill the valence's well hydrogen only gets 2 from rule 1 okay or rule 2 okay
so only gets 2 so it's done it doesn't get any lone pairs oxygen gets an octet 2 4 6 8
10 12 so we're out of electrons with carbon does not have an octet it only has 6 electrons from
three bonds 3 times 2 is 6, 2 electrons and every bond 2 and every lone pair so we've got
to give multi bonds to carbon by converting a lone pair on oxygen into a double bond to
carbon we do that we get this structure it has no formal charge on any atom and we are done