TWEED RIVER HIGH SCHOOL
2006
PRELIMINARY CHEMISTRY
Unit 3
Water
Part 2
The wide distribution and importance of water on Earth is a
consequence of its molecular structure and hydrogen bonding.
 Construct Lewis electron do structures of water, ammonia and
hydrogen sulfide to identify the distribution of electrons.
 Compare the molecular structure of water, ammonia, and hydrogen
sulfide, the differences in their moleculare shapes and in their
melting and boiling points.
 Process information from secondary sources to graph and compare
the boiling and melting points of water with other similar sized
molecules.
Copy Table 13.5, p233 text and plot a suitable graph from this data.
Lewis Electron DOT Structures
1. Count Electrons
Lewis electron dot structures show the valence electrons for each atom.
You don't need to worry about the total number of electrons, only those in
the outer shells.
1
2. Electron dot for water
The following diagram shows the formation of the Lewis electron
dot diagram for water.
The diagram on the right hand side shows the Lewis electron dot
structure for water. The ‘x’ shows the electron from hydrogen that
is being shared with the oxygen electron.
Lewis Dot Structure for Ammonia
2
Lewis Dot Structure for Hydrogen Sulfide
 Describe hydrogen bonding between molecules.
 Identify water as a polar molecule
 Describe the attractive forces between polar molecules as dipoledipole forces
Electronegativity
 The electronegativity of an element is a measure of the ability of
the atom of that element to attract bonding electrons towards itself
when it forms compounds.
 If the electronegativities of two elements differ by more than about
1.5, then the elements will tend to form ionic compounds.
 Electronegativities increase from left to right across any given
period and they increase from bottom to top in any particular
group.
 Fluorine is the most electronegative element. Francium is the least
electronegative element.
3
Electronegativities for some common elements.
Element
Electronegativity
(Pauling Scale)
H
2.2
He
0
Li
1.0
Be
1.6
B
2.0
C
2.6
N
3.0
O
3.4
F
4.0
Ne
0
Na
0.9
Mg
1.3
Al
1.6
Si
1.9
P
2.2
S
2.6
Cl
3.2
Ar
0
K
0.8
Ca
1.0
Fr
0.7
4
Intermolecular Forces

Forces exist between molecules that are not chemical bonds.
1. Dipole-Dipole interactions

Molecules in which the bonding electrons are unevenly shared between the
bonded atoms are called polar molecules and the bonds are called polar
(covalent) bonds.

A pair of equal and opposite charges separated in space is called a dipole.

The positive and negative charges interact between molecules to form an intermolecular attraction.

Molecules which have atoms of significantly different electronegativity’s form
dipoles. Theses dipoles are permanent. For example Hydrogen Chloride, HCl
is a polar molecule. Because Chlorine has a higher electronegativity than
Hydrogen, most electrons will be around the Chlorine end of the molecule. As
electrons are negatively charged, the chlorine end of the molecules has a small
net negative charge. Therefore, the hydrogen end of the molecule has a small
net positive charge.
This is shown by the following diagrams:
1. Electron distribution cloud diagram – shows most electrons are around the Cl
atom.
5
2. The following diagram shows the net charge on the molecule.
The δ (small Greek letter delta) means “small”. The δ+ means a small positive
charge.
The ‘dashed’ line between the chlorine and the hydrogen shows the dipole –
dipole interaction caused by the electrostatic attraction between positive and
negative charges. This is the intermolecular force.
6
Hydrogen Bonding
Hydrogen bonds are specific dipole-dipole intermolecular attractions that form
between H – O, H – N and H – F. These intermolecular forces are stronger than other
dipole - dipole interactions.
NOTE: They are called Hydrogen bonds but they are not chemical bonds. They are
intermolecular forces.
Hydrogen bonding in water:
7
Dispersion Forces
The dispersion force is the weakest intermolecular force. The dispersion force is a
temporary attractive force that results when the electrons in two adjacent atoms
occupy positions that make the atoms form temporary dipoles. This force is
sometimes called an induced dipole-induced dipole attraction. Dispersion forces are
the attractive forces that cause nonpolar substances to condense to liquids and to
freeze into solids when the temperature is lowered sufficiently.
Because of the constant motion of the electrons, an atom or molecule can develop a
temporary (instantaneous) dipole when its electrons are distributed unsymmetrically
about the nucleus.
A second atom or molecule, in turn, can be distorted by the appearance of the dipole
in the first atom or molecule (because electrons repel one another) which leads to an
electrostatic attraction between the two atoms or molecules.
Dispersion forces are present between any two molecules (even polar molecules)
when they are almost touching.
Molecular Size
Dispersion forces are present between all molecules, whether they are polar or
nonpolar.

Larger and heavier atoms and molecules exhibit stronger dispersion forces
than smaller and lighter ones.
8


In a larger atom or molecule, the valence electrons are, on average, farther
from the nuclei than in a smaller atom or molecule. They are less tightly held
and can more easily form temporary dipoles.
The ease with which the electron distribution around an atom or molecule can
be distorted is called the polarizability.
Dispersion forces tend to be:



stronger between molecules that are easily polarized.
weaker between molecules that are not easily polarized.
Explain the following properties of water in terms of its
intermolecular forces:
- surface tension
-
viscosity
-
boiling and melting points
Surface Tension
 Water molecules at the surface of a beaker, are not surrounded by
other water molecules in the same way as those molecules in the
centre of the beaker. Surface tension results from the molecules on
the surface of water (or any liquid) having an overall attractive force
downwards into the rest of the water. This downwards force creates
a tension on the surface of the water, so that it behaves like a tightly
stretched skin.
 Water has a high surface tension because of its ability to form many
hydrogen bonds.
Draw Fig 13.19, p232 text
9
Viscosity
 The viscosity of a liquid is a measure of its resistance to flow.
 When a liquid flows the molecules slide over one another. If the
molecules have strong intermolecular attractions and are long and thin
and easily tangle the liquid will have a high viscosity, e.g. tar and
honey. If the molecules have weak intermolecular attractions and are
smaller molecules then the liquid has a low viscosity.
 Viscosity decreases as temperature rises.
 Gases have the lowest viscosity.
 Viscosity relates directly to the strength of the forces between
molecules and the size of the molecules. These forces determine how
easily the molecules move past each other.
 The viscosity of water is greater than many other similar liquids, e.g.
petrol, because the intermolecular forces are much stronger in water
than those in other liquids.
Melting and Boiling Points
 The melting and boiling points of water are much higher than
molecules of similar size. This is because of the strong hydrogen
bonds in water and the fact that each water molecule is hydrogen
bonded to four other water molecules.
 It takes a greater amount of heat energy to increase the kinetic energy
of the molecules, which is required to break the relatively strong
hydrogen bonds.
 Identify data and process information from secondary sources to
model the structure of the water molecule and the effects of forces
between water molecules. (You can refer to diagram from Part 1).
10