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.