So far we have looked at covalent molecules. But how do we know that they are covalent? The answer
comes from electronegativity. Each element (except for the noble gases) has an electronegativity
value.
Electronegativity is a measure of how strongly an atom pulls a shared electron pair
towards it. The table below shows the electronegativities of the some of the elements.
For a full list of electronegativities see the periodic table at the front of the book. On this
periodic table the electronegativity values are given in the top right corner. Do not confuse
these values with the other numbers shown for the elements. Electronegativities will always be
between \(\text{0}\) and \(\text{4}\) for any element. If you use a number greater than
\(\text{4}\) then you are not using the electronegativity.
Depending on which source you use for electronegativities you may see slightly different
values.
Element
Electronegativity
Element
Electronegativity
Hydrogen (\(\text{H}\))
\(\text{2,1}\)
Lithium (\(\text{Li}\))
\(\text{1,0}\)
Beryllium (\(\text{Be}\))
\(\text{1,5}\)
Boron (\(\text{B}\))
\(\text{2,0}\)
Carbon (\(\text{C}\))
\(\text{2,5}\)
Nitrogen (\(\text{N}\))
\(\text{3,0}\)
Oxygen (\(\text{O}\))
\(\text{3,5}\)
Fluorine (\(\text{F}\))
\(\text{4,0}\)
Sodium (\(\text{Na}\))
\(\text{0,9}\)
Magnesium (\(\text{Mg}\))
\(\text{1,2}\)
Aluminium (\(\text{Al}\))
\(\text{1,5}\)
Silicon (\(\text{Si}\))
\(\text{1,8}\)
Phosphorous (\(\text{P}\))
\(\text{2,1}\)
Sulfur (\(\text{S}\))
\(\text{2,5}\)
Chlorine (\(\text{Cl}\))
\(\text{3,0}\)
Potassium (\(\text{K}\))
\(\text{0,8}\)
Calcium (\(\text{Ca}\))
\(\text{1,0}\)
Bromine (\(\text{Br}\))
\(\text{2,8}\)
Table 3.2:
Table of electronegativities for selected elements.
Electronegativity
Electronegativity is a chemical property which describes the power of an atom to
attract electrons towards itself.
The concept of electronegativity was introduced by Linus Pauling in 1932, and this
became very useful in explaining the nature of bonds between atoms in molecules. For
this work, Pauling was awarded the Nobel Prize in Chemistry in 1954. He also received
the Nobel Peace Prize in 1962 for his campaign against above-ground nuclear testing.
The greater the electronegativity of an atom of an element, the stronger its attractive pull on
electrons. For example, in a molecule of hydrogen bromide (\(\text{HBr}\)), the
electronegativity of bromine (\(\text{2,8}\)) is higher than that of hydrogen (\(\text{2,1}\)),
and so the shared electrons will spend more of their time closer to the bromine atom. Bromine
will have a slightly negative charge, and hydrogen will have a slightly positive charge. In a
molecule like hydrogen (\(\text{H}_{2}\)) where the electronegativities of the atoms in the
molecule are the same, both atoms have a neutral charge.
Worked example 9: Calculating electronegativity differences
Calculate the electronegativity difference between hydrogen and oxygen.
Read the electronegativity of each element off the periodic table.
From the periodic table we find that hydrogen has an electronegativity of
\(\text{2,1}\) and oxygen has an electronegativity of \(\text{3,5}\).
Calculate the electronegativity difference
\(\text{3,5} - \text{2,1} = \text{1,4}\)
Textbook Exercise 3.7
Calculate the electronegativity difference between: \(\text{Be}\) and
\(\text{C}\), \(\text{H}\) and \(\text{C}\), \(\text{Li}\) and
\(\text{F}\), \(\text{Al}\) and \(\text{Na}\), \(\text{C}\) and
\(\text{O}\).
\(\text{Be}\) and \(\text{C}\): \(\text{2,5} - \text{1,5} =
\text{1,0}.\)
\(\text{H}\) and \(\text{C}\): \(\text{2,5} - \text{2,1} =
\text{0,4}.\)
\(\text{Li}\) and \(\text{F}\): \(\text{4,0} - \text{1,0} =
\text{3,0}.\)
\(\text{Al}\) and \(\text{Na}\): \(\text{1,5} - \text{0,9} =
\text{0,6}.\)
\(\text{C}\) and \(\text{O}\): \(\text{3,5} - \text{2,5} =
\text{1,0}.\)
Electronegativity and bonding (ESBMF)
The electronegativity difference between two atoms can be used to determine what type of
bonding exists between the atoms. The table below lists the approximate values. Although
we have given ranges here bonding is more like a spectrum than a set of boxes.
Electronegativity difference
Type of bond
\(\text{0}\)
Non-polar covalent
\(\text{0}\) - \(\text{1}\)
Weak polar covalent
\(\text{1,1}\) - \(\text{2}\)
Strong polar covalent
\(>\) \(\text{2,1}\)
Ionic
Note that metallic bonding is not given here. Metals have low electronegativities and
so the valence electrons are not drawn strongly to any one atom. Instead, the
valence electrons are loosely shared by all the atoms in the metallic network.
Non-polar and polar covalent bonds (ESBMG)
It is important to be able to determine if a molecule is polar or non-polar since the
polarity of molecules affects properties such as solubility,
melting points and boiling points.
Electronegativity can be used to explain the difference between two types of covalent bonds.
Non-polar covalent bonds occur between two identical non-metal atoms,
e.g. \(\text{H}_{2}\), \(\text{Cl}_{2}\) and \(\text{O}_{2}\). Because the two atoms
have the same electronegativity, the electron pair in the covalent bond is shared
equally between them. However, if two different non-metal atoms bond then the shared
electron pair will be pulled more strongly by the atom with the higher
electronegativity. As a result, a polar covalent bond is formed where
one atom will have a slightly negative charge and the other a slightly positive charge.
This slightly positive or slightly negative charge is known as a partial charge. These
partial charges are represented using the symbols \({\delta }^{+}\) (slightly positive)
and \({\delta }^{-}\) (slightly negative). So, in a molecule such as hydrogen chloride
(\(\text{HCl}\)), hydrogen is \(\text{H}^{\delta^{+}}\) and chlorine is
\(\text{Cl}^{\delta^{-}}\).
The symbol \(\delta\) is read as delta.
Polar molecules (ESBMH)
Some molecules with polar covalent bonds are polar molecules,
e.g. \(\text{H}_{2}\text{O}\). But not all molecules with polar covalent bonds
are polar. An example is \(\text{CO}_{2}\). Although \(\text{CO}_{2}\) has two polar
covalent bonds (between
\(\text{C}^{\delta^{+}}\) atom and the two \(\text{O}^{\delta^{-}}\) atoms), the
molecule itself is not polar. The
reason is that \(\text{CO}_{2}\) is a linear molecule, with both terminal atoms the
same, and is therefore symmetrical. So there is no difference in charge between the two
ends
of the molecule.
Polar molecules
A polar molecule is one that has one end with a slightly
positive charge, and one end with a slightly negative charge. Examples
include water, ammonia and hydrogen chloride.
Non-polar molecules
A non-polar molecule is one where the charge is equally
spread across the molecule or a symmetrical molecule with polar bonds.
Examples include carbon dioxide and oxygen.
To determine if a molecule is symmetrical look first at the atoms around the central
atom. If they are different then the molecule is not symmetrical. If they are
the same then the molecule may be symmetrical and we need to look at the shape
of the molecule.
We can easily predict which molecules are likely to be polar and which are likely to be
non-polar by looking at the molecular shape. The following activity will help you
determine this and will help you understand more about symmetry.
Polar and non-polar molecules
The following table lists the molecular shapes. Build the molecule given for each
case using jellytots and toothpicks. Determine if the shape is symmetrical.
(Does it look the same whichever way you look at it?) Now decide if the molecule
is polar or non-polar.
State whether hydrogen (\(\text{H}_{2}\)) is polar or non-polar.
Determine the shape of the molecule
The molecule is linear. There is one bonding pair of electrons and no lone
pairs.
Write down the electronegativities of each atom
Hydrogen: \(\text{2,1}\)
Determine the electronegativity difference for each bond
There is only one bond and the difference is \(\text{0}\).
Determine the polarity of each bond
The bond is non-polar.
Determine the polarity of the molecule
The molecule is non-polar.
Worked example 11: Polar and non-polar molecules
State whether methane (\(\text{CH}_{4}\)) is polar or non-polar.
Determine the shape of each molecule
The molecule is tetrahedral. There are four bonding pairs of electrons and no
lone pairs.
Determine the electronegativity difference for each bond
There are four bonds. Since each bond is between carbon and hydrogen, we only
need to calculate one electronegativity difference. This is:
\(\text{2,5} - \text{2,1} = \text{0,4}\)
Determine the polarity of each bond
Each bond is polar.
Determine the polarity of the molecule
The molecule is symmetrical and so is non-polar.
Worked example 12: Polar and non-polar molecules
State whether hydrogen cyanide (\(\text{HCN}\)) is polar or non-polar.
Determine the shape of the molecule
The molecule is linear. There are four bonding pairs, three of which form a
triple bond and so are counted as \(\text{1}\). There is one lone pair
on the nitrogen atom.
Determine the electronegativity difference and polarity for
each bond
There are two bonds. One between hydrogen and carbon and the other between
carbon and nitrogen. The electronegativity difference between carbon and
hydrogen is \(\text{0,4}\) and the electronegativity difference between
carbon and nitrogen is \(\text{0,5}\). Both of the bonds are polar.
Determine the polarity of the molecule
The molecule is not symmetrical and so is polar.
Electronegativity
Textbook Exercise 3.8
In a molecule of beryllium chloride (\(\text{BeCl}_{2}\)):
What is the electronegativity of beryllium?
\(\text{1,5}\)
What is the electronegativity of chlorine?
\(\text{3,0}\)
Which atom will have a slightly positive charge and
which will have a slightly negative charge in
the molecule? Represent this on a sketch of the
molecule using partial charges.
Beryllium will have a slightly positive charge and
chlorine will have a slightly negative charge.
Is the bond a non-polar or polar covalent bond?
Polar covalent bond. The electronegativity difference
is: \(\text{3,0}-\text{1,5} = \text{1,5}\). The
bond is strongly polar.
Is the molecule polar or non-polar?
Beryllium chloride is linear and symmetrical.
Therefore it is a
non-polar
molecule.
Complete the table below:
Molecule
Difference in
electronegativity
between atoms
Non-polar/polar covalent
bond
Polar/non-polar
molecule
\(\text{H}_{2}\text{O}\)
\(\text{HBr}\)
\(\text{F}_{2}\)
\(\text{CH}_{4}\)
\(\text{PF}_{5}\)
\(\text{BeCl}_{2}\)
\(\text{CO}\)
\(\text{C}_{2}\text{H}_{2}\)
\(\text{SO}_{2}\)
\(\text{BF}_{3}\)
Molecule
Difference in
electronegativity
between atoms
Non-polar/polar covalent
bond
Polar/non-polar
molecule
\(\text{H}_{2}\text{O}\)
\(\text{3,5}-\text{2,1}=\text{1,4}\)
Polar covalent bond
Polar molecule. Water has a bent or
angular shape.
\(\text{HBr}\)
\(\text{2,8}-\text{2,1}=\text{0,7}\)
Polar covalent bond
Polar molecule. Hydrogen bromide is
linear.
\(\text{F}_{2}\)
\(\text{4,0}-\text{4,0}=\text{0}\)
Non-polar covalent bond
Non-polar molecule.
\(\text{CH}_{4}\)
\(\text{2,5}-\text{2,1}=\text{0,4}\)
Polar covalent bond
Non-polar molecule. Methane is
tetrahedral.
\(\text{PF}_{5}\)
\(\text{4,0}-\text{2,1}=\text{1,9}\)
Polar covalent bond
Non-polar molecule. Phosphorous
pentafluoride is trigonal
bypramidal and symmetrical.
\(\text{BeCl}_{2}\)
\(\text{3,0}-\text{1,5}=\text{1,5}\)
Polar covalent bond
Non-polar molecule. Beryllium
chloride is linear and
symmetrical.
\(\text{CO}\)
\(\text{3,5}-\text{2,5}=\text{1,0}\)
Polar covalent bond
Polar molecule. Carbon monoxide is
linear, but not symmetrical.
\(\text{C}_{2}\text{H}_{2}\)
\(\text{2,5}-\text{2,1}=\text{0,4}\)
Polar covalent bond
Non-polar molecule. Acetylene is
linear and symmetrical.
\(\text{SO}_{2}\)
\(\text{3,5}-\text{2,5}=\text{1,0}\)
Polar covalent bond
Polar molecule. Sulfur dioxide is
bent or angular and is not
symmetrical.
\(\text{BF}_{3}\)
\(\text{4,0}-\text{2,0}=\text{2,0}\)
Polar covalent bond
Non-polar molecule. Boron trifluoride
is trigonal pyramidal and not
symmetrical.
