POLAR AND NONPOLAR MOLECULES
•Molecular polarity can be determined by symmetry. Symmetry can
be defined as a balance or regular arrangement of the atoms within
the molecule.
•If a molecule is symmetrical in shape, it is said to be…
•NONPOLAR-----without poles.
•If a molecule is not regular, it is said to be…
•POLAR-----containing (+) and (-) poles.
Let’s take a look at some specific
molecules…
This line is
called the
Axis of
Symmetry
Cl-|-Cl
We can draw a line down the
middle of this molecule.....
The left side looks like the right. The molecule is symmetrical and is
therefore a NONPOLAR MOLECULE.
The molecule…
H-|-Cl
The left contains Hydrogen and the right contains Chlorine.
Nonsymmetrical and therefore POLAR.
H
H C H
H
Is this a nonpolar or a polar molecule?
POLAR
NONPOLAR
CH4 is methane gas that we use to fuel our Bunsen Burners. It is a
very symmetrical molecule and is therefore a nonpolar molecule.
INCORRECT
This molecule is symmetrical
O
H
H
Is this molecule nonpolar or polar?
POLAR
NONPOLAR
Water molecules are angular in shape due to the two bonded
hydrogens and the two unbonded pairs of electrons on the oxygen.
This angular shape, along with the electronegativity of the oxygen,
causes the molecule to be polar.
CORRECT!
O=C=O
Is this molecule nonpolar or polar?
POLAR
NONPOLAR
INCORRECT
Carbon dioxide has two double covalent bonds that form a linear
molecule. It is very symmetrical and therefore nonpolar.
This molecule is symmetrical
N
H
H
H
POLAR
Polar or nonpolar, that is the question?
NONPOLAR
Ammonia has a trigonal pyramidal shape due to the unbonded
pair of electrons on the nitrogen. It has definite polar
characteristics because it is not totally symmetrical. Yet it is not
as strong of a polar bond as water due to the lower
electronegativity of the nitrogen atom.
CORRECT!
SYMMETRICAL
ON
N
NONPLOAR
A
I
YM
E C
S M TR L
POLAR
Molecules that are polar are
also said to be dipoles. Dimeans 2. –pole means it has (+)
and (-) electrostatic poles.
Thus the name DIPOLE.
Because dipoles have charged ends, they can
attract each other. This is called…DIPOLEDIPOLE ATTRACTION.
Molecules that are polar are
also said to be dipoles. Dimeans 2. –pole means it has (+)
and (-) electrostatic poles.
Thus the name DIPOLE.
Because dipoles have charged ends, they
can attract each other. This is called
Dipole-dipole attraction. The stronger
the dipoles, the stronger the force of
attraction.
Because a dipole has charged ends, it
can attract the ions in an ionic
compound also. This is called the iondipole attraction.
Network Solids
• Network Solids are also called covalent
crystals. Covalent bonds extend from 1 atom
to another in one repeating pattern.
• Because of this network of covalent bonds,
such substances do not have distinct
molecules. The entire mass of a network solid
is considered a single macromolecule.
A few Network Solids:
• Diamonds: These are a large group of carbon
atoms bonded in a three-dimensional crystal.
• Silicon dioxide: This is silicon and 2 oxygen
atoms covalently bonded, also known as
mineral quartz.
• Silicon Carbide: Carbon atoms covalently
bonded with silicon, also known as
carborundum. (used as an abrasive)
HYDROGEN BONDING
Compounds such as water and ammonia have hydrogen atoms that are bonded
to smaller atoms of relatively high electronegativity.
Let’s look at water as an example.
The high attraction of oxygen for
hydrogen’s electron partially
exposes the nucleus of hydrogen.
In other words, electrons that
form the bond between the
hydrogen and oxygen move a little
closer to the oxygen. This causes
the hydrogen to be partially
positive and the oxygen to be
partially negative.
HYDROGEN BONDING
Notice how the water molecules are attracted to each other. This is an
example of hydrogen bonding.
MORE POWER
The fluorine atom has a higher electronegativity (a greater attraction for an
electron) than an oxygen atom. Therefore the electrons in an H-F bond are
pulled closer to the fluorine and this makes the hydrogen bond between H-F
molecules stronger.
London Dispersion Forces
At any instant in time, the electrons in a particle may be unequally distributed.
This means one side of the molecule will have a charge for just an instant.
This slight charge will affect the neighboring molecule and force that
molecules electrons into an unequal pattern as well.
This produces a slight attraction between the molecules.
London Dispersion Forces
• The slight attraction between molecules can
then affect the other molecules near them.
• This is the weakest force of the Van der Waal’s
forces.
• This weak force can be increased by an
increase in the number of electrons.
• This weak force will be decreased if the
molecules are further apart.