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I think we are on part four now, and this is just after the slide on ozone
O3
and this illustrates what i was mentioning in that last video about
nitrate
here having three different structures
and as it says in the bottom the real nitrate
is analyzed to have three
equal bonds between nitrogen and oxygen
and it's really four electron pairs being shared between nitrogen and three
other atoms
As it says it's like a one and a third bond
can't draw a one and a third bond, so we just draw these different possibilities
with the realization that nitrate is
somewhere in the middle
and for a molecule or an ion, the more resonance structures week can draw
the more stabilizing that is
and so we want to give the complete picture by drawing all the possibilities
and for the molecules we see they may only have one structure
that will work
but once you start drawing double bonds and asking yourself if you could draw
them in different places to make it work
to make that octet rule work then you should draw all those other
possibilities and for nitrate it's these three
and once we draw those structures it's not always true that they are equally equivalent
for
describing what the molecule really looks like
in nitrate all three of those structures are essentially the same
one double bond between nitrogen and oxygen
and then the two single bonds
here's three resonance forms for an ion called cyanate and it would be
worth your practice to verify these on your own
you start with one oxygen, one carbon, one nitrogen and negative charge, that's
sixteen electrons altogether and that leads to these three possibilities. The octet
rule satisfied across the board for each one
and as it says these are not equivalent
and so
is one of these better than the other? Is one of them
more like what cyanate really looks like?
the answer is yes
but we need
a little more help figure that out
turns out that third structure is the one that's most commonly used to
represent cyantate because it is the best one
it has to do a calculating these things called formal charges
and it's just a little recipe for counting electrons according to that
formula
and as it says the bottom there
the most stability is when you have zero formal charge or when they are as low as
possible
we'll get back to the cyanate example but here is
a calculation for carbon monoxide
both atoms have the octet rule satisfied so that's good, but a little bit of
instability is due to the fact that the carbon and oxygen each have a formal
charge
negative on carbon positive on oxygen
this is not the same as negatives and positives that we use to talk about
electrons being transferred. it's really just a bookkeeping method to alert us to
structures that are more stable than others
and carbon monoxide is a highly reactive molecule
and the formal charges suggest that should be true
carbon dioxide by contrast is much more stable and if you do those calculations
on these atoms you get zero across the board
so you want to get used to calculating such things for every atom in each
structure
so that
we'll know which atoms have
minimum formal charges and if we can draw a resonance structures where there
are zero formal charges
that's the one we're going to tend to focus on
Here is what we look for with formal charges
once we identify where they are we
pick out the structures that again minimize these
plus and minus one is better than plus and minus two
and it says negative charges should be on the most electionegative atom
and in the periodic chart that's the ones up in the top right hand corner
like nitrogen
oxygen and chlorine, expecially. They can handle negative charge's better than say
carbon can
And so resonance structures that have that to be true
are better
here's the answer key to doing formal charges for cyanate
and our answer there says that that structure on the right is best
because notice the only formal charge is a negative one on oxygen and oxygen can
handle that negative charge better than carbon
better than nitrogen
on the one on the far left is probably the second best structure because it
also only has a single formal charge
the one in the middle yikes! we've got
formal charges on two of the three atoms
as you do this you encounter some trends like these where you can recognize
on sight which patterns give zero formal charges
you should commit this type of stuff to memory. It will come in handy later on