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Covalent Bonding
This occurs when non metal atoms bond together. They
share pairs of electrons to give oneanother complete
outer shells. Here covalent molecules are formed.
The simplest example is hydrogen. It has only one
electron and the first shell can only hold two.
H
H
Two separate
hydrogen atoms
Combine
H
H
to give
A hydrogen molecule, H2
Combine hydrogen and chlorine covalently
Hydrogen and chlorine are both non metals. They are both composed of
diatomic molecules. They combine as follows:
H2(g) +
x
H
+
Cl2(g) --------> 2HCl(g)
Cl
H
x
Cl
A hydrogen chloride molecule
This is called a “dot and cross” diagram. It
is used to show clearly which electrons
come from which element.
Do both elements have a Noble gas configuration in the product
molecule?
Combine hydrogen and oxygen covalently
If hydrogen and oxygen combine in this way, how many atoms
of each element do we need? Work it out using a diagram.
H
produces
H
O
H
H
O
A water molecule
We often represent covalent structures as “stick” diagrams or
more correctly as structural formulae. Each shared pair of
electrons is known as a single covalent bond. It is represented
as a single straight line in a structural formula.
O
H
H
Combine hydrogen and carbon
How many hydrogen atoms do we need to give carbon a
Noble gas structure in a simple hydrocarbon?
H
C
H
H
C
H
H
In this space draw the complete dot and
cross diagram, the structural formula, the
chemical formula and the name of the
compound
H
H
C
H
Methane
H
CH4
Double covalent bonds
Some covalent molecules seem to have too few atoms to work with single
covalent bonds. They have four shared electrons in some of their bonds.
There are two well known examples, the alkenes and carbon dioxide. In the
space below, draw the dot and cross diagrams and structural formulae of
Ethene (C2H4)
and
H
H
C
C
H
C
O
O
H
H
H
C
H
Carbon dioxide (CO2)
C
H
Are these molecules saturated or unsaturated?
O
C
O
The properties of simple covalent materials
By simple molecules, we mean small molecules which are not linked up
into giant structures. Examples are CO2, H2O, CH4, H2.
Compared to ionic compounds (which are giant structures), do they have high
or low melting and boiling points?
NaCl m.pt. = 801oC
CH4 m.pt. = - 183oC
LiF m.pt = 876oC
H2 m.pt. = -259oC
Why is this? Is it because covalent bonds are weak and easier to break than
ionic bonds?
H
H
No!!!. The covalent bonds inside
C
C
H
the simple molecules are strong
H
H
H
H
H
H
and are not broken when they are
C
melted or even boiled. What does
H
H
break are weak intermolecular
H
bonds called Van der Waals bonds
Because of their low m.pts and b.pts., many simple covalent materials are
gases and liquids at room temperature.
Are simple covalent materials electrical conductors?
They are insulators. They never conduct, not as solids or as liquids.
Why is this?
They contain no charged particles which are free to move. All of their
electrons are either in bonds or part of the inner shells of atoms.