A CUP OF WATER
Until recently it has been completely
impossible to ‘see’ a molecule. Realistically
speaking, it still is – for most people, most of
the time. So, in order to try to visualize what
is happening, we need to use models.
Models give us the best and most direct view
of the molecular world. Modelling is also an
excellent tool for learning about chemical
theory and atomic structure.
Structure of Water
[Figure 2]
The water molecule, visualized three different ways: ball-and-stick model, space-filling model, and
structural formula with partial charges.
Water is a simple molecule consisting of one oxygen atom bonded to two
different hydrogen atoms. Because of the higher electronegativity of the
oxygen atom, the bonds are polar covalent (polar bonds). The oxygen atom
attracts the shared electrons of the covalent bonds to a significantly greater
extent than the hydrogen atoms. As a result, the oxygen atom acquires a
partial negative charge (�−), while the hydrogen atoms each acquire a partial
positive charge (�+). The molecule adopts a bent structure because of the
two lone pairs of electrons on the oxygen atom. The H-O-H bond angle is
about 104.5° (slightly smaller than the ideal 109.5° of an sp3 hybridized
atomic orbital).
The bent shape of the water molecule is critical because the polar O-H bonds
do not cancel one another and the molecule as a whole is
polar. Figure below illustrates the net polarity of the water molecule. The
oxygen is the negative end of the molecule, while the area between the
hydrogen atoms is the positive end of the molecule.
[Figure 3]
Water is a polar molecule, as greater electron density is found around the more electronegative oxygen
atom.
Polar molecules attract one another by dipole-dipole forces as the positive
end of one molecule is attracted to the negative end of the nearby
molecule. In the case of water, the highly polar O-H bonds results in very
little electron density around the hydrogen atoms. Each hydrogen atom is
strongly attracted to the lone-pair electrons on an adjacent oxygen
atom. These are called hydrogen bonds and are stronger than conventional
dipole-dipole forces.
[Figure 4]
A hydrogen bond is the attraction between a lone pair of electrons on the oxygen atom of one molecule
and the electron-deficient hydrogen atom of a nearby molecule.
Because each oxygen atom has two lone pairs, it can make hydrogen bonds
to the hydrogen atoms of two separate other molecules. Figure below shows
the result – an approximately tetrahedral geometry around each oxygen atom
consisting of two covalent bonds and two hydrogen bonds.
[Figure 5]
As a result of two covalent bonds and two hydrogen bonds, the geometry around each oxygen atom is
approximately tetrahedral.
Purpose: To build geometrically accurate water molecules and model H-bonding
between them in 2 and 3 dimensions.
Part 1
Materials:
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Red Plasticine
White Plasticine
Tooth picks
Clear plastic Cup
Scale
Procedure: You will be making a cup of water. You will be constructing 5
molecules of water and showing the H-bonding between them
Key
Hydrogen = White
Oxygen = Red
 The atomic mass of Hydrogen in 1.008 and will be represented by 1 g of
white clay.
 The atomic mass of Oxygen is 15.999 and will be represented by 16g of Red
clay.
1) Weigh 10 pieces of white clay with a mass of 1.0g each. Form round
spheres with them.
2) Weigh 5 pieces of red clay with a mass of 16.0g each. Form round spheres
with them.
3) When all 15 spheres are formed, weigh them on the scale. It should have a
mass of 90.0g.
4) Construct 5 molecules of water. Remember the bond is 104.5˚ .
5) Attach the 5 molecules of water together with tooth picks showing the Hbonding between them. (Remember tetrahedral shape).
6) Place the water molecules in a clear plastic cup.
Key
Hydrogen = White
Oxygen = Red
Voila! Cup of water
Each H2O molecule should have the same mass (make sure on the scale).
This means that each H atom has the same mass and each O atom has the same
mass prior to assembly.
A Cup of Water!!!
Part 2
Paper models
Materials:
 Red paper
 White paper
1. Make 2 red circles and 4 white circles (sizes provided)
2. Place all the electrons, protons and neutrons on the paper provided to
show the adequate bonding.
3. Stick to the wall.
4. Show the H-bonding between molecules.
Summary

Water is a molecular compound consisting of polar molecules that have a
bent shape.

The oxygen atom acquires a partial negative charge while the hydrogen
atom acquires a partial positive charge.
Review
1. What type of bond exists in a water molecule?
2. Which part of the molecule has a partial positive charge?
3. Which part of the molecule has a partial negative charge?
4. How do water molecules interact with one another?
Why does cohesion and adhesion occur in water?
Water has some unusual properties due to its hydrogen bonds. One property is
cohesion, the tendency for water molecules to stick together. Cohesion causes water to
have surface tension, the property that makes water molecules stick together and form
droplets. Another important physical property of water is adhesion. In terms of water,
adhesion is the bonding of a water molecule to another substance, such as the sides of
a leaf's veins. This grip-like characteristic that water molecules form causes capillary
action, the ability of a liquid to flow against gravity in a narrow space. Adhesion and
capillary action are necessary to the survival of most organisms. It is the mechanism
that is responsible for water transport in plants through roots and stems, and in animals
through small blood vessels.