- [Instructor] Let's do one more example
of constructing a Lewis diagram
that might be a little bit interesting.
So let's say we wanted to construct the Lewis structure
or Lewis diagram for xenon difluoride.
So pause this video and have a go of that.
All right, now let's work through this together.
So first step,
we just have to account for the valence electrons.
Xenon right over here.
It is a noble gas.
It has eight valence electrons.
One, two, three, four, five, six, seven, eight
in that fifth shell.
It's in the fifth period.
So it has eight valence electrons.
And then fluorine,
we have looked at fluorine multiple times,
we know that it has seven valence electrons.
One, two, three, four, five, six, seven
in that second shell.
And we have two of these fluorines.
So two times seven.
And then this gives us a total of
eight plus 14 valence electrons
which gets us to 22 valence electrons in total.
Now the next step, and we've done this multiple times,
in multiple videos now,
is we would try to draw the structure
with some single covalent bonds
and we would put xenon as our central atom
because it is less electronegative than fluorine.
So let's put a xenon there.
And let's put two fluorines on either side.
So fluorine there
and a fluorine there.
And let's set up some single covalent bonds.
And so how many of our valence electrons
have we now accounted for?
Well two in that bond and then two in that bond.
So we've accounted for four.
So minus four valence electrons.
We now have a total of 18 valence electrons.
Now the next step is we wanna allocate them
to our terminal atoms and try to get them to a full octet.
Each of these fluorines already have two valence electrons
that they are sharing.
So we need to give each of them six more.
So two, four, six.
Two, four, six.
So I've just allocated 12 more valence electrons.
So minus 12 valence electrons
means that we still have six valence electrons
left to allocate.
And there's only one place
where we can allocate those left over six valence electrons
and that's at the central atom.
At the xenon.
So let's do that.
So, two, four, and six.
And there you have it.
We have the Lewis diagram, the Lewis structure
for xenon difluoride.
Now what's interesting here is
our fluorines they have an octet of valence electrons.
But what's going on with xenon?
Xenon has two, four, six, eight,
10 valence electrons hanging around.
So this is one of those examples
of an exception to the octet rule
where we go beyond eight valence electrons
which is possible for elements
in the third or higher period.