in which we will explore how bond polarity affects the polarity of a molecule
so what is polarity? First molecules are neutral; the entire molecule
has an equal amount of protons and electrons; with that in mind
if the negative charge, the electrons in a molecule, are evenly distributed around
the molecule, then it is not polar. however if the negative charge is not
evenly distributed, then the molecule is polar.
Water is a well-known polar molecule.
electrons are moving very fast but the more electronegative oxygen
pulls electrons away from the less electronegative hydrogen.
In this color-coded illustration the more red area has a greater concentration of electrons
so the oxygen side of water is more negative while the
blue area has a lower concentration of electrons so it is more positive.
water has an uneven distribution of electrons, it is a polar molecule.
but why does this occur?
we will take a closer look at water and a variety of other molecules
to see what determines whether a molecule is polar, including its
and we will go through many examples of this
we will go through several examples
demonstrating these concepts
and then put them in a flowchart.
Let's take a close look at water and see why it is a polar molecule.
first of all it is bent,
meaning it does not
have a symmetrical shape
this is important,
and here we will define symmetrical as having
the central atom here is oxygen
and both hydrogen's are on one side of the oxygen.
which makes it non-symmetrical. the hydrogens are not
evenly distributed around the oxygen.
next we look at bond polarity
by the difference in bonded atoms' electronegativities.
in general electronegativity values increase across
the periodic table from left to right and from bottom to top.
we see here that hydrogen has an electronegativity of 2.1
and oxygen has an electronegativity of 3.5
the difference between 3.5 and 2.1 is fairly large,
meaning the hydrogen-oxygen bond has a large polarity
which is shown with an arrow pointing towardthe more electronegative atom
in this case oxygen
if we view the bond polarities as vectors and add them together
the resulting vector, or arrow, indicates the polarity of the molecule.
the blue arrow is called a dipole,
note these are partial charges,
not the full charge that an ion would have
let's look at a less familiar molecule sulphur difluoride with sulfur in the center.
the Lewis structure shows two lone pairs on the sulfur
and so it is a bent, and therefore non-symetrical molecule.
the electronegativity differences in the fluorine-sulfur bonds show a large bond polarity, but this time
pointing toward the outside atoms, the more electronegative fluorines.
adding the bond polarity vectors again gives a large dipole,
with sulfur partially positive and the fluorines partially negative.
next let's look at carbon dioxide with carbon in the center.
the Lewis structure shows each carbon-oxygen bond being a double bond.
the shape of CO2 is linear triatomic
which is a symmetrical shape.
the oxygens are evenly distributed around the carbon.
there is a fairly large electronegativity difference in the carbon-oxygen bond.
So will there be a dipole? what will happen when we add the bond polarity vectors?
that's right! they add up to zero, they cancel each other out so there is no dipole.
I hope you can see the importance of symmetry here.
the symmetry is why the bond polarities cancel.
now let's make the outside atoms different. How will that affect polarity?
here we see the electronegativity differences in the bonds, and thus the bond polarities are different.
what will be the result of adding them together?
that's right they add up to a nonzero vector.
SCO is a polar molecule with the oxygen side being more negative
the sulfur side being more positive.
Let's do a couple more examples,
and then put all this information together in a flow chart.
boron trifluoride with boron in the center
the fluorines are equidistant from each other at 120.
the electronegativities show a large difference in the B-F bond
resulting in a large bond polarity in each bond.
however if we add up the bond polarity vectors they add up to zero.
the bond polarities cancel due to symmetry
and so there is no dipole.
let's do something interesting.
let's replace one of the fluorines in BF3 with a chlorine, BF2Cl.
but the electronegativity differences for each bond are no longer the same.
The B-Cl electronegativity difference is only half that of the B-F bond.
adding the bond polarity vectors results in a non zero vector.
which is the dipole.
everything we have been looking at so far
has been the result of combining bond polarities with the molecule shape
and identity of the outside atoms to determine molecule polarity.
Let's put that in a flow chart.
you begin by looking to see if any bonds are polar
and if not then you cannot have a polar molecule.
for the purposes of determining molecular polarity
we consider an electronegativity difference greater than zero to be polar.
now if there are polar bonds then you look at the
symmetry of the outside atoms which is determined by the shape of the molecule.
symmetrical shapes are
outside atoms are evenly distributed around the central atom.
non-symmetrical shapes are
and we include diatomic as a special case
which we will get back to momentarily.
let's keep that list in the corner for reference.
the rest of the flow chart summarizes the examples we have just looked at.
if polar bonds are not symmetrically distributed,
then the molecule is polar.
if they are symmetrically distributed
then you have to look at the outside atoms
if they are the same
that means the bond polarities cancel
but if the outside atoms are different
that means the electronegativity differences are giving different magnitudes of bond polarities
and so they do not cancel.
let's take a quick look at diatomic molecules,
our examples being HCl and Cl2
For HCl is polar and so the molecule is polar. for Cl2 there is no bond polarity.
lastly we will take a brief look at how changes in
a tetrahedral molecule affects polarity.
In methane everything is symmetrical
bond polarities cancel
and it is nonpolar. But if we substitute a fluorine
for one hydrogen
then both the magnitude and direction of that bond's polarity changes
the bond polarity vectors add up to a large dipole so CH3F is polar.
changing another hydrogen to fluorine has the same effect on bond polarity.
again creating a dipole
but with a slightly different direction
changing a third hydrogen to fluorine
we get the same effect,
the polarity changes direction
but it is still polar.
What will happen to the polarity
if we change the fourth hydrogen to fluorine?
yes you're right
since all the outside atoms are the same
the symmetry results in bond
polarity vectors adding to zero
they cancel each other and
carbon tetrafluoride
is a non-polar molecule.