HAZEL SIVE: Let's
talk about bond types.
And I'm going to distinguish
four, two of which
are going to be a
kind of a joint class.
There are ionic bonds,
covalent, hydrogen.
And I'll tell you that hydrogen
bonds are very big in biology.
You may not be familiar
with those at all,
and you need to be.
And then I'm going
to put together
van der Waals and non-polar
or hydrophobic bonds
in one class, although
strictly speaking,
that's not quite fair.
From strongest to weakest,
the ionic and covalent bonds,
and this bottom class are
the weakest, with hydrogen
somewhere in the middle.
Let's take a look at some
of these chemical bonds
and see what we
can say about them.
OK, so I've summarized
for you here the notion
that ionic bonds are attractions
between atoms that involve
oppositely charged ions.
And so in this molecule
that I've shown you here,
there is a negatively charged
part and a positively charged
part, and they are attracted
to one another very strongly,
and that holds the
molecule together.
In a covalent
bond, the electrons
are shared by two atoms,
and this contrasts, again,
with the ionic bond,
where electrons
move to one part of the
molecule or to another part
of the molecule, or to a
completely different molecule,
and the added--
the molecule is held
together with the electron
split completely away
from one of the atoms.
In a covalent bond, in
contrast, the electrons
are shared by two atoms.
And I've circled there a bond
between a carbon and an oxygen.
It's actually two bonds.
That's why there are two lines.
And those are two
covalent bonds.
The hydrogen bond is one, as I
mentioned, that is key to life.
Hydrogen does not have
high electronegativity.
It's electropositive,
if you like,
and so it often has a
partial positive charge.
This allows it to be attracted
to an electronegative atom,
which will have a
partial negative charge.
These two differential
dipoles, the positive
and the negative
dipole, will then
hold together two
different molecules
and form a bond between them.
The hydrogen bond is
shown by a dotted line.
It is not a covalent bond.
It is an electrostatic bond.
And it always involves
hydrogen, hence, the name.
If you look at
molecules in biology,
and you're not sure
what kind of bond
it is, if you see
hydrogen, and then
you see some kind of
electronegative atom,
like oxygen or nitrogen,
you can be fairly sure
that you are looking
at a hydrogen bond.
And as we'll explore
a bit more, and you'll
explore in further
biology courses,
hydrogen bonds are unbelievably
important in biology.
And then this last class of
non-polar and van der Waals
bonds or forces occur
between non-polar molecules
where in at least
one form, there
may be very transient dipoles
between different molecules
that holds the
molecules together
as a transient positive and
a transient negative dipole.
And now, I'd like you
to predict the bond.
Go to your next
assignment and see
if you can predict the bond
based on the characteristics
that I've just given you.
