Water as a Polar
Molecule
TAKS: Objective 4
TEKS: 8D
The Polarity of Water
 Water
has a simple
molecular structure. It
is composed of one
oxygen atom and two
hydrogen atoms. Each
hydrogen atom is
covalently bonded to
the oxygen via a
shared pair of
electrons.
Recall
 What
is electronegativity????
chemical property that describes the ability
of an atom (or, more rarely, a functional
group) to attract electrons (or electron
density) towards itself
 Looking at water, which element has a
higher electronegativity?
OXYGEN
Since oxygen has a higher electronegativity
than hydrogen, the electrons of the
molecule tend to group closer to the
oxygen than to the hydrogen atoms. This
means that even though the molecule as a
whole is neutral, the oxygen side has a
slight negative charge while the hydrogen
side has a slight positive charge.
Therefore, water is said
to be a "polar" molecule,
which means that there is
an uneven distribution of
electron density.
The electrostatic attraction
between the partial positive
charge near the hydrogen
atoms and the partial
negative charge near the
oxygen results in the
formation of a hydrogen
bond
Hydrogen bond [H-bond]
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A hydrogen bond is a non-covalent bond
between a partial negative charge and a
partial positive charge.
Hydrogen bonds tend to be weak.
Hydrogen bonds tend to be transient.
Hydrogen bonds are very numerous which
somewhat offsets their weak and transient
nature.
On average each water molecule in liquid
water is hydrogen bonded to 3.4 other water
molecules.
Many other unique properties of
water are due to the hydrogen bonds.

For example, ice floats because hydrogen bonds
hold water molecules farther apart in a solid than
in a liquid, where there is one less hydrogen bond
per molecule. Therefore, the density decreases.
High Boiling Point
 On
average, each water
molecule interacts with
about four others.
 In
water vapor, the molecules are too far
apart for hydrogen bonding to occur. This
means that in order for us to boil water we
must break all the hydrogen bonds in
liquid water. Breaking those bonds takes
energy, thus the high boiling point for
water.
Cohesion


The attraction of one water molecule to
another resulting from hydrogen bonding.
By placing a drop of water on a surface you
can directly observe cohesion in the
resistance that water droplet shows to
wetting, i.e., water clumps up in a pile
despite being a liquid, rather than spreading
out over the surface.
Adhesion

Similar to cohesion except adhesion
involves the attraction of a water molecule to
a non-water molecule.
How is adhesion taking
place in this image??
Surface Tension
 The
water molecules
on the surface have
partners for hydrogen
bonding only within the
liquid; above the water
surface there are no
more molecules
available for hydrogen
bonding. This means
that molecules at the
surface experience a
net force pulling them
inward.
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Properties of Water
1. a. Draw the structure of water. Include the
partial charges of each atom.
b. Why is water considered to be a polar
molecule?
2. a. What enables neighboring water molecules to
hydrogen-bond to one another?
b. How many hydrogen bonds can each water
molecule form?
3. Explain the difference between adhesion and
cohesion. Give an example of each.
4. What is surface tension? Give an example.
5. Explain why water is considered to have a high
boiling point.