Download Properties of Water Due to Polar Covalent Bonding - CRHS

Polar Covalent Bonding Within the
Water Molecule Explains Four
Physical Properties of Water
Observation 1:
H2O
Water has a concave miniscus.
Observation 2:
Water molecules stay
together in a stream.
Observation 3:
A falling water stream is attracted to
an electrically charged object.
H2O
+
charged object
Observation 4:
ice
H2O
Ice floats.
H O H
The Lewis dot structure of water cannot explain
these four observations.
1+
8+
1+
The oxygen atom is more electronegative than the hydrogen
atoms. As a result, the four pairs of shared and unshared electrons
are closer to the oxygen atom than the hydrogen atoms. This
unequal sharing of pairs of electrons in a covalent bond is
referred to as a polar covalent bond.
8+
1+
104.5o
1+
The water molecule is bent because of the arrangement
of these four pairs of electrons in three - dimensional
space.
8+
1+
1+
The four pairs of electrons repel one another. As these
pairs of electrons are arranged in three-dimensional
space, rather than two-dimensional space, their actual
positions are not illustrated properly here.
H
O
H
This perspective diagram is a three-dimensional
representation of the water molecule on two dimensions.
The electron pairs are shown to be as far apart from one
another as possible. The unshared electron pair in the lower
left is enlarged to illustrate the idea they are coming
forward out of the page. The small pair of unshared
electrons is receding behind the plane of the page. The
hydrogen and oxygen symbols are in the plane of the page.
H
The four electron pairs are located
at the corners of a triangular
pyramid called a tetrahedron. The
oxygen nucleus is located in the
very centre of the pyramid. The
two pairs of shared electrons
define two vertices of the
tetrahedron while the two pairs of
unshared electrons are found at the
other two vertices.
O
H
1+
8+
104.5
1+
o
The tetrahedral arrangement of the electron pairs in
three-dimensional space causes the water molecule
to be bent.
1+
8+
104.5o
1+
The bend in the molecule and the greater
electronegativity of the oxygen atom causes the
electron pairs to be closer to the oxygen atom
than the hydrogen atom.

_
+
8+
1+
104.5o
 + 1+
The greater electronegativity of the oxygen atom and
the bend in the water molecule causes an uneven
distribution of negative charge in the water molecule.
The oxygen end of the molecule is slightly negative
(δ-) while the hydrogen ends of the molecule are
slightly positive (δ+). Water is said to have positive
and negatively charged ends or poles. Thus water is
said to be a polar molecule.
hydrogen bonds form
between water
molecules
The small negative charge on the
oxygen atom of one water molecule
will attract the small positive charge
of the hydrogen on another water
molecule forming what is called a
hydrogen bond. This attraction or
hydrogen bonding between adjacent
water molecules explains the four
observations.
Observation 1: Water forms a concave meniscus.
H
O
H
O
H
O
H
O
H
H
H
O
O
H
H
H
O
O
H
O
H
H
O
H
H
O
H
O
O
H
O
H
H
O
H
H
H
H
H
H
H
O
H
H H
H
H
H
H
H
H
H
O
H
O
H
O
H
H
O
Water molecules are not only attracted to
each other but also to the walls of the
container.
The
attraction
water
molecules have for the walls of the
container causes them to “climb” up the
wall. This attraction of water molecules
to the walls of the container is called
adhesion. The attraction water molecules
have for each other is termed cohesion.
Together, adhesion and cohesion are
responsible for the formation of the
meniscus.
Observation 2: Water molecules stay together in a stream.
H H
O
O
H H
H
H
O
O
H
H
H
H
O
O
H
H
H
O
H
O
H
H
O
H
H H H
O
H
O
H H
O
O
H
H
H
H
H O
O
H
H
O
O
H
H
H
H
H
O
H
O
H
HO
H
In flowing water the hydrogen bonds between
adjacent water molecules are transient;
forming, breaking and reforming as water
molecules slide past one another in the liquid
state. This attraction or cohesion between
water molecules is responsible for holding
water molecules together in a drop and a
stream.
H
Observation 3: A falling water stream is attracted to an
electrically charged object.
H
O
H
O
As water molecules stream past a
positively charged object, water
molecules rotate because the
negative oxygen end of the water
molecule is attracted to the
positive charge. The entire stream
is pulled towards the charged
object because of the transient
hydrogen bonds between adjacent
water molecules
O
H
H
H
O
H H
HH
O
HO
H
H
O
+
H OH
O
H
H
H O
O
H
H
O
H
H
O
H
H
H
H
O
H
O
H
H
O
H
O
H
H
H
H O
O
H
O
H
H
H
H
H
Observation 4: Ice floats.
Water molecules are fixed in position
in the ice crystal by hydrogen bonds
between water molecules. The
hexagonal arrangement of six water
molecules in ice takes up more space
than the same six molecules in the
liquid state. Hence ice is less dense
than water and floats. Water is rare
in that the solid state is less dense
than its liquid state.
For most
substances the solid state is more
dense than the liquid state.