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welcome to part two of chapter one this slide begins a series
illustrating how we show ionic bonds forming between atoms
if you recall from general chemistry
ionic bonds are
metal to nonmetal bonds
positive metal cations
negative nonmetal anions
and although organic chemistry focuses a lot on carbon and hydrogen and
oxygen
that is non metal elements bonding to each other
we will see some ionic formulas
and that's the easier place to start
when it comes to
talking about bonding
and one of the things that's true about molecules
that have ionic bonds is that
they always form
compounds in which the atoms appear in certain ratios sodium chloride is the
famous example
and this formula out top is indicating equal numbers of sodium and chlorine
that's because what's over here in yellow is what's driving all of these reactions
to take place
metals like to get rid of any valence electrons they have, and for sodium
there's only one of them
non metals like to gain electrons usually to have a set of eight
chlorine is much happier as a
negatively charged chloride ion at the bottom
and so both adams are accomplishing this task of getting rid of partially filled
valence levels
and so they tend to do that
and metals love to give electrons away so
there's plenty of examples. This next slide shows magnesium and oxygen
accomplishing the same task
is just a matter of transferring two electrons to make that happen
and that's really the key word for ionic bonds is transfer of electrons
metals react with non metals like oxygen very easily
because they have a desire to get rid of their valence electrons
non metals are looking for
eight-- that's going to be a rule that that helps us a lot as we go along this
looking for a collect ron's around non metals
this next example illustrates why magnesium chloride has
one magnesium and two chlorines in its formula
why two chlorines?
well one chlroine isn't enough, magnesium will be happy to get rid of
one of its valence electrons to a chlorine
but magnesium wants to get rid of both of those valence electrons not just one
of them so it ends up finding a second chlorine to finish that off
and so when that is done
you have to have two chlorides for every one magnesium in order for the chlorines
to each end up with eight and for magnesium can empty out its valence
level
so these atoms are accomplishing the same task
as the sodium and chlorine
here's sodium and oxygen doing something very similar
you need to two sodiums for each one oxygen in order for the oxygen to end up with
eight here
and in each case i'm starting with how many valence electrons these atoms would
have before they bond
and then at the bottom showing
where the electrons end up
and that's pretty much the ionic bonding story
For organic chemistry though this question is more important for us: how do atoms
bond if there aren't any metals to
get the electrons from
hydrogen doesn't exist as individual atoms but rather is pairs of
hydrogens
and it's because they need to be able to share
there one electron with a neighbor
in order to become more stable
and for hydrogen it only needs two electrons to be happy
that makes it a simple case
but two electrons for each hydrogen gives it
the arrangement that helium has and for hydrogen that's what it's going for.
For other non medals though, like i say, eight's the magic number
chlorine can
take electrons from sodium but if there aren't metals around, then it will share
with itself it, will share with hydrogen
it will share with lots of other non metals but always for the purpose of
getting eight around them
these examples show water
and ammonia--why their formulas are what they are
same kind of thing
and there are examples like these
throughout chapter one
you see double bonds and triple bonds whenever atoms can't get the job done
simply by sharing single pairs of electrons. But notice that the oxygen and
the nitrogen atoms at the bottom
they end up with eight electrons on each atom
that's really the key
a slide act like this tells us why
we might care about all that bonding because it helps us explain
behavior
as atoms bond to one another