Here we'll start looking at hydrogen bonds. Now
hydrogen bonds always sound like the strongest
of the bonds. I don't know maybe it has something
to do with hydrogen bomb but it's nothing like that.
Hydrogen bonds are actually the weakest of the
biological bonds that we'll be looking at
in this course. So, let's start by looking at
water. Again water is a covalently bonded molecule
itself. But between water molecules we're
actually going to see hydrogen bonding.
So, taking a look at water we see that oxygen has
a valence electron. Six valence electrons
and thus a valence of two. It would like two
more electrons. Before, we looked at oxygen
sharing with oxygen to create a double
covalent bond but oxygen has other options.
In this case, we'll have two bonds with hydrogens.
So each of those two hydrogens contribute
one shared electron and now oxygen is happy with
its octet rule. Eight electrons in the outer shell.
And the two hydrogen ions are also satisfied
by sharing their electrons with the oxygen.
So they only need two in their outer shell. So, we
have two covalent bonds from hydrogen to oxygen
and hydrogen to oxygen.
So because oxygen is more electronegative we have
a polar molecule. What I mean by this is oxygen
has little bit more draw for the electrons that
hydrogen is sharing with it. It's kind of greedy.
And so because it's pulling electrons more
towards its center, oxygen ends up with a slightly
negative charge. So it's more electronegative
which means it has more draw for the negative electrons.
So the other end of the molecule would be the
hydrogen end of the molecule. And the hydrogen
end of the molecule has a slightly positive charge.
Because the oxygen end of the water molecule
has a slightly negative charge and the hydrogen
end of the water molecule has a slightly positive charge,
the opposite ends of the molecule are attracted
to other water molecules such that the hydrogen
end of one water molecule is slightly attracted
to the negative end of, the oxygen end of
the water molecule. So in this case, this is
why water will associate and become liquid
at room temperature because the water molecules
are in constant motion changing their associations.
So it's less strong of a positive negative difference
than an ionic bond but a slight charge difference
When we heat water, it will evaporate and that's
because we're increasing the molecular motion
and that positive negative attraction is not
quite enough to hold the water molecules together.
So, water evaporates. When we freeze water, it
becomes solid because those associations
between positive and negative becomes stronger
because cooler temperatures mean less molecular motion.