in this video we're gonna discuss the octet rule
and lewis structures for simple
just elements and for more complex compounds
if you recall from the periodic table the noble gases which are in Group
8 a are the least reactive elements on the periodic table
they are the least reactive because they have a full set
a valence electrons they have 8 valence electrons
so all the other elements on the periodic table
basically want to be like a noble gas and have 8 valence electrons and that's
where the octet rule comes in
atoms on the periodic table will gain, lose,
or share valence electron so that they achieve
a full set of valence electrons which is 8
however there are a couple of exceptions to the octet rule
and those are hydrogen and helium since hydrogen and helium are on the first
row of the periodic table a full set
valence electrons consists of only two valence electrons
so hydrogen and helium will only have two all the other elements on the
periodic table will try to
achieve 8 valence electrons, we can represent the number of valence electrons in
an element
using Louis symbols we will use a dot to represent the number of valence electrons
lithium only has one valence electron because its in group 1a on the periodic
table
carbon has four valence electrons it's in group 4a
so that group number one it's a representative element tells you how
many valence electrons it has
oxygen is in group 6a has six valence electron so we have six dots around
the symbol for oxygen and neon is a noble gas it has 8 valence electrons
so we have 8 dots around it
those are the Lewis symbols for just elements
we can make more complex lewis structures for a whole compound that
shows how the
electrons are shared within that compound so that every element
in the compound can achieve its octet or its full set of electrons
the steps for writing a lewis structure are written on the board here
and basically there are three or four steps depending on the structure
or on the compound the first step is to sum up your valence electrons for
all of the atoms in the compound okay so you're going to go to the periodic table
and you're going to see what group number
each element is in and you're gonna add up all the valence electrons
the second step will be to draw the skeleton structure which consists of
basically putting
the first atom thats in the chemical formula in the center
and put the other atoms around the outside there is an exception to that
rule as well hydrogen can never be in the center
once you have that drawn you're going to connect all of your
outter elements to the center atom with the line which represents a bond
then we have to distribute our remaining electrons
around so that everybody gets their full set
of 8 except for hydrogen which is only gonna have 2
at that point we'll check and make sure that everybody has their octet
or there maximum number if not
we go to Step 4 and that's where we may have to make multiple bonds
so we're going to do a few examples and
draw some lewis structures for
a few different compounds the first compound we're going to do
is methane CH4 okay so here's our formula
first step sum up the valence electrons
carbon is in group 4a so it has four valence electrons
hydrogen is in group 1a it has one valence electron
there are four hydrogens in the compound some then multiply it by four
add them up we have 8 electrons
so we have 8 electrons to distribute and share amongst all the
atoms in that compound so that everybody has a full set
of valence electrons step 2 drawing the skeleton structure
the first atom in the formula goes in the center
the other atoms go around the outside
and we're gonna connect each of the outer atoms to the center with a bond
we use a line to represent a bond, a bond
represents two electrons a pair of electrons
so so far in our skeleton structure we have used 2
4, 6, 8 electrons to connect the outer elements
to the center element we're going to subtract that from our total number of
electrons
and that tells us that we have used all of our electrons
so now we don't have any more to distribute so we're going to check and
make sure that everybody has their full set of valence electrons
hydrogen is an exception and only needs two
if we look at this hydrogen atom it has its two electrons because it sharing
these with carbon
so it has two this hydrogen has two
this hydrogen has two and this hydrogen has two so all the hydrogens are happy
now we have to look at the center carbon this carbon
gets use of all of the
bonded the electrons in the bonds
so it has 2, 4, 6, 8
electrons around it that's its full set so
this is the complete lewis structure for methane
let's do another example
phosphorus trichloride
first step sum up the valence electrons phosphorus is in group 5a
so it has five valence electrons chlorine is in Group 7a
so it has 7 valence electrons
we have three chlorine atoms multiply it by three
when we add it up we should have 26 electrons
so step 2
our skeleton structure first atom is going to go in the center
the remaining atoms go around the outside
connect each of them with a bond
the bond represents two electron so we have used 2
4, 6 electrons I'm gonna subtract it from our total number of electrons so we have
20
electrons left to share to distribute third step
distribute the electrons starting with
the outer atoms so
let's pick a chlorine atom this chlorine atom already has two electrons
so it needs six more to complete its octet
so we're going to and six more electrons
this chlorine already has two
were gonna and six more so than it completes its octet
same for this has two
we add six more so we have used 6, 12
18 electrons we have two
remaining two electrons remaining we have already given electrons
to all the outer atoms to fulfill the octet any that are left over at this
point
will go to the center atom so I'm going to give these two
to phosphorus subtract we have used all of our electrons
we've distributed them all so now we have to check make sure everyone has the octet
if you look at this chlorine it has all eight
this chlorine has all 8, this one has 8
checked the center atom phosphorus has 2
4, 6 from bonds and the 2 that are a lone pair
so it has 8 as well so
this is the complete electron or lewis structure for
phosphorus trichloride we're going to do one more example
that takes us down to step 4
SO2 sulfur dioxide first step
sum up your valence electrons sulfur is in group 6a
so has six oxygen is in group 6a
it has six and there are two of them so we have 18 electrons to share
draw our skeleton structure
S two Os and you can put the Os anywhere you wish
attach each with the bond so we have used 2
4 electrons so far so we have 14 electrons remaining
we need to distribute them starting with the outer atoms first
so this one needed 6 more to get
8 this oxygen needed six more to get
eight so we've used 12 we have two left over
the leftovers always go to the center atom
you could put it on top or bottom doesn't matter I've given those two to
sulfur
we have no more electron so we've distributed all the electrons
it's time to check does everybody have 8 this oxygen has all
8 this oxygen has all 8
sulfur has 2, 4
6 does not have all eight electrons
so we are not done sulfur has to have all
8 what happens at this point is that one of the
outer atoms has to share more electrons
so the oxygen for example
on either side doesn't matter which side you choose
they have all of these lone pairs that aren't being shared
so we can take one of these pairs and move it
in between the S and
O so it's being shared now the sulfur
gets to use those electrons along with the oxygen
so you have formed a double bond
and the final structure
would look like this: the lone pair that was here has moved in between it's being
shared
now the sulfur has 2, 4,
6, 8 electrons and everyone is happy
the key here the forming multiple bonds
only happens if the center atom does not have its full octet
after step 3: okay so you
that is always a last resort to form multiple bonds
so with those three examples that we did
I think you have a pretty good understanding of how to draw lewis
structures for
any type of covalent compound