HAZEL SIVE: Another key
concept that we will discuss
is electronegativity.
Electronegativity
refers to the attraction
of an atom for electrons.
And in biology, we
need to distinguish
two kinds of molecules
that are different based
on their electronegativity.
And I'm going to abbreviate
electronegativity
for the rest of
this board as en.
And the types of molecules
that we need to discuss
are called polar,
where polar molecules
have an unequal
electronegativity, unequal en.
And these molecules
are always hydrophobic
or tend to be hydrophobic
based on having one charged
portion of the molecule.
The other type, the opposite,
are non-polar molecules
where there is equal
electronegativity.
And these tend to
be hydrophobic.
OK.
Let's look at some
polar molecules.
Whether a molecule
is polar or not polar
depends on two
things, whether it
has got polar bonds where
the electrons are unequally
distributed.
And it also depends on the
geometry of the molecule.
But we won't deal
too much about that.
I've put it in for
completeness' sake.
So here is water.
And you can see that there is--
I've drawn an unequal
electron distribution.
This is called a dipole.
And the dipole is because
the oxygen of the water
is very electronegative.
It pulls the
electrons towards it
and it depletes the electrons
somewhat from the hydrogens
that it is bonded to.
So you get a molecule
where the oxygen
is somewhat negative because
of its high electronegativity.
And the hydrogens are somewhat
positive because of their lower
electronegativity.
And this is indicated
by those deltas.
Delta minus means
that the atom is
more negative than an atom
that's indicated delta plus.
So delta negative is the oxygen.
Delta plus is the hydrogen.
The geometry of
the molecule also
is such that these
dipoles don't cancel out.
And so you get overall a
molecule that is polar overall.
Here are two examples.
One is polar and
one is not polar.
The top example is propane,
which has the chemical formula,
as you can see, C3H8.
It is just carbons
and hydrogens.
And carbons and hydrogens,
neither of which
are particularly
electronegative atoms,
neither have a tendency to pull
electrons more than the other
and so the bonds and the
molecule are overall non-polar.
There is an equal
distribution of electrons
throughout the molecule.
On the other hand,
isopropanol, you can see,
has got a hydroxyl
group added to it.
And there is that oxygen which
is highly electronegative.
It will be pulling
electrons away
from the carbons and the
carbon and the hydrogens.
And therefore you get a
partial negative charge
or a more negative
character in the hydroxyl
and the oxygen part
of the molecule.
And this molecule is polar.
And you can get a hint as to
whether a molecule is polar
or not by whether or not it
has an electronegative atom
like oxygen that is
associated with it.
And now we're going
to take a pause.
And you are going
to do an assignment
to practice whether or
not you can recognize
polar and non-polar molecules.
