problem uh all right so i need to start rearranging
rearranging
my lone pairs into double bonds so i'll
start with one
move this guy down again i'm already
always going to start with a lone pair from
from
the guy that's already got electrons
that already has a full octet and try to
share them with the atom that doesn't
have a full octet so let me just go
ahead and erase
that and redraw it and then i'm going to
check my
octet again nitrogen still has my still
has an octet carbon
2 4 6 still doesn't have an octet so i'm
going to take another lone pair off
the nitrogen and share that again so i
will erase this
just for clarity and draw that out
now i'm going to check this again 2 4 6
8 nitrogen still has an octet
2 4 6 8. now carbon has an octet
um so my last step as always is to
put some brackets around this and redraw
it uh sorry and add the at the charge
i can if i want to redraw this a little
bit more neatly
with um and this is just a thing that
we'll talk about when we get to the geometry
geometry
section but we tend to put uh we tend to
try to distribute the
lone pairs sort of evenly around
our atoms it doesn't matter um that's
just sort of
because it looks nicer and it's a little
bit more realistic that the electrons
are further away from the other
electrons as we'll talk about
in the geometry section but this is
all right so now we're going to move on
to talking about exceptions to the octet
rule and how that changes
or doesn't change our our process of drawing
drawing
lewis structures so we talked before
about the fact that some molecules are
going to have odd electrons
they're going to have an odd number of
valence electrons which means that just
by definition not all atoms will have an octet
octet
because it's not possible um so what's
going to happen is we're going to have
one unpaired electron
and we need to give that to the less
electronegative atom
that is more likely to have an unfilled octet
octet
so we'll just do this exactly the same
way we start off with
figuring out how many valence electrons
we have so nitrogen has five valence electrons
electrons
oxygen has six so we wind up with 11
electrons all right then our step two is
to draw our skeleton structure
we don't have a central atom so we'll
just go ahead and connect our
two atoms with a single bond and then we
will do our electron bookkeeping and
subtract two electrons
so we are left with nine electrons
all right so our next step is to assign
these lone pairs which
here is where it gets a little weird
right we've got basically four
lone pairs plus one electron so we need
to assign these to
our molecule and we'll start with the
more electronegative atoms and we'll
move to the less electronegative atoms
so we've got four lone pairs to
distribute we need to fill octets oxygen
already has two
so we'll give it six more
so that's three of our lone pairs so
we'll give nitrogen our last lone pair
and we will also give it the unpaired electron
electron
so uh that's all the electrons we've got
to work with we are done
so then step five is to check our
valence uh our octets oxygen two four six
six
eight has our octet nitrogen two four
five doesn't have an octet and in fact
it's pretty far off from an octet
um and so with these ones it's not so
much meeting the octet
but trying to get to seven um we can't
get to eight but we can get to seven
so let's go ahead and reassign one of
these lone pairs
to be a double bond so when we do that
we will find uh now we'll double check
again we've got two four six
eight still hasn't still has an octet
and then two four
six seven so nitrogen doesn't
technically have an octet here but it is
close as
as we can get uh given that it it is
stuck with that odd electron
um and this is not this is uh it's not a charged
charged
particle it's not an ion or anything so
we don't put it in brackets we just
leave it alone
all right um next we will talk about atoms
atoms
that are electron deficient so uh for
example bf3
where uh somebody in here might not have
a filled octet uh this can happen with
groups two
and thirteen um
all right so we'll start off exactly the
same way we're gonna start off by
uh calculating the number of valence
electrons boron has
three valence electrons and fluorine
each one has seven
and then we need to multiply that by
three because we have uh
three fluorines and so when we finish
off our
math we will find that this is going to
have 24
electrons all right our step two
is to draw a skeleton structure boron
goes in the middle because it is less
electronegative than fluorine
and then we will surround it with the three
three
fluorines all right and then we will do
our electron bookkeeping
two four six subtract six electrons so
we've got eighteen
electrons left over which means six
uh nope nope it does not mean six it
all right so now we need to go ahead and
assign them
uh so step three is to assign them
assign them to our external
um external atoms so we'll go ahead and
do that
uh so each of these fluorines has two so
far so it can
have it needs three more lone pairs
we'll go ahead and do that so
one two three
one two three
one two three all right so that is
actually all of our electrons so
2 4 6 8 10 12 14 16 18
20 22 24. so that's it we're done this
is all of our electrons we don't have
anybody left over to give to the boron
so normally our uh our step four we
don't have anybody left step five we
would we would be checking our
our octets and we would see that boron
doesn't have an octet it's only got six
electrons around it
and we would be choosing one of these
guys to give to make a double bond
um and it turns out that when you're
dealing with a group
uh 2 or 13 element
you're just not going to do that you're
just going to leave leave that lone pair
alone and let that boron
stay with six electrons um this is what
you find experimentally you do find that
this uh this atom or that sorry this
this molecule does act like a boron with
three single bonds around it um we can
tell with bond length then we can also
look at reactivity
this is a super reactive molecule it it
does act like the boron
is electron deficient
all right so our last kind of
exceptions to the octet rule are
hypervalent molecules
where the central atom can exceed the octet
octet
so as i mentioned before only elements
in the third and higher period
uh can have this happen and it's because
they have mtd orbitals
so phosphorus in the third energy level
doesn't have any electrons in the 3d uh orbitals
orbitals
because they they don't fill up until
the fourth energy level but they do
exist and so
um basically they can because there are
those empty d orbitals
um elements in the third row and below
can exceed the octet
all right so let's go ahead and talk
about how you would draw
some of this so if we look at
phosphorous and phosphorus pentachloride
five chlorines
it's exactly the same process we start
off with figuring out how many electrons
um there are how many valence electrons
we've got
so phosphorus lives just below nitrogen
in the periodic table it's going to have
five valence electrons and then we have
chlorines which each have seven valence
electrons and there are five of them
and when we figure this out we find out
we have 40 electrons
in our structure then we will go ahead
and draw out a skeleton structure and
we'll put the least electronegative atom
in the center and then everybody else
attached to it by a single bond
here phosphorus is less electronegative
than chlorine and we'll go ahead and
draw this
and yeah we're going to go ahead and put
five chlorines around
our phosphorus um
so here's our five chlorines around our
phosphorus um it feels weird but it it
this is what actually uh happens in uh
in reality
all right so now i need to do my
electron bookkeeping two four six
eight ten subtract 10 electrons
which is 15 lone pairs all right and
then i'm going to start assigning them
to my
terminal external more electronegative
atoms first i'll go ahead and do this in green
green
so i'm just going to keep track of how
many electrons i use up
one two three four
five six
7 8 9
10 11 12
13 14 15. all right that's it that's as
many electrons as i've got
um i should go ahead and double check my
octets um and i i can do that so
look at the chlorines each of these
chlorines has eight electrons around it
uh they are all fulfilled uh phosphorus
technically has 10 electrons around it so
so
it is exceeding its octet um it is
fulfilled i don't need to rearrange
anybody to
try to get that phosphorus more electron
all right so then we're going to look at
one more example and this is
uh this is um xenon
tetrafluoride xenon is a noble gas so it
seems very strange
xenon actually can make uh stable
compounds under certain conditions and
this is one of them
so we're gonna exactly the same way we
would we're just gonna treat this
um like like any other compound we're
going to go ahead and figure out how
many valence electrons this structure
should have
um xenon has eight valence electrons and
the four fluorines each have seven so
we'll do
seven times four um and so when we
finish off our electron math we're going
to find
that this structure has 36 electrons
all right then we're going to put the uh
least electronegative atom in the center
which turns out to be
xenon and then we will arrange the
fluorines around
it attached by single bonds
and then step three after i do my electron
electron
bookkeeping so subtract one two three four
four
four lone pairs or eight electrons so
subtract eight electrons
we wind up with 28 electrons left over
which is 14 millimeters
all right so now we're going to start
assigning these lone pairs to my
terminal or external
atoms so 14 lone pairs so let's go
1 2 3 4
5 6 7
8 9 10
11 12. all right so i have used up 12
lone pairs
i have two lone pairs left
all right um step four is to assign lone
pairs to
my central atom so even though xenon
technically has
eight electrons here i have two more
electrons i need to deal with
and i'm going to put them around my
xenon so
this is what it looks like fluorine we
can check our octet so fluorine
atoms each have eight electrons and the xenon
xenon
atom has 12. um so and
yeah again this is this is what we
actually see we can we can
we can look at some properties and
understand that this is is in fact
likely the correct structure for this
molecule um it's important to note we
can extend the octet right we've
extended it
uh phosphorus here has 10 uh 10
electrons around it instead of eight
here xenon's got 12. we don't generally
see anything above 12 when we're looking
at main group elements so
um six lone pairs or six six
regions of density is pretty much the
max for
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