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Electronegativity

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 0 and 4 for any element. If you use a number greater than 4 then you are not using the electronegativity.

Tip:

Depending on which source you use for electronegativities you may see slightly different values.

Table 1: Table of electronegativities for selected elements.

Element

Electronegativity

Element

Electronegativity

Hydrogen ()

2,1

Lithium ()

1,0

Beryllium ()

1,5

Boron ()

2,0

Carbon ()

2,5

Nitrogen ()

3,0

Oxygen ()

3,5

Fluorine ()

4,0

Sodium ()

0,9

Magnesium ()

1,2

Aluminium ()

1,5

Silicon ()

1,8

Phosphorous ()

2,1

Sulfur ()

2,5

Chlorine ()

3,0

Potassium ()

0,8

Calcium ()

1,0

Bromine ()

2,8

Definition 1: Electronegativity

Electronegativity is a chemical property which describes the power of an atom to attract electrons towards itself.

Interesting Fact:

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 (), the electronegativity of bromine (2,8) is higher than that of hydrogen (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 () where the electronegativities of the atoms in the molecule are the same, both atoms have a neutral charge.

Example 1: Calculating electronegativity differences

Question

Calculate the electronegativity difference between hydrogen and oxygen.

Answer

Read the electronegativity of each element off the periodic table.

From the periodic table we find that hydrogen has an electronegativity of 2,1 and oxygen has an electronegativity of 3,5.

Calculate the electronegativity difference

Exercise 1:

Calculate the electronegativity difference between: and , and , and , and , and .

and :

and :

and :

and :

and :

Electronegativity and bonding

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.

Figure 1
Image
Table 1
Electronegativity differenceType of bond
0Non-polar covalent
0 - 1Weak polar covalent
1,1 - 2Strong polar covalent
2,1Ionic

Tip:

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

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. , and . 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 (slightly positive) and (slightly negative). So, in a molecule such as hydrogen chloride (), hydrogen is and chlorine is .

Tip:

The symbol is read as delta.

Polar molecules

Some molecules with polar covalent bonds are polar molecules, e.g. . But not all molecules with polar covalent bonds are polar. An example is . Although has two polar covalent bonds (between atom and the two atoms), the molecule itself is not polar. The reason is that 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.

Definition 2: 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.

Definition 3: 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.

Tip:

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.

Activity 1: 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.

Table 2

Geometry

Molecule

Symmetrical

Polar or non-polar

Linear

   

Linear

   

Linear

   

Bent or angular

   

Trigonal planar

   

Trigonal planar

   

Trigonal pyramidal

   

Tetrahedral

   

Tetrahedral

   

Trigonal bipyramidal

   

Trigonal bipyramidal

   

Octahedral

   

Octahedral

   
Molecule Polarity
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Example 2: Polar and non-polar molecules

Question

State whether hydrogen () is polar or non-polar.

Answer

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: 2,1

Determine the electronegativity difference for each bond

There is only one bond and the difference is 0.

Determine the polarity of each bond

The bond is non-polar.

Determine the polarity of the molecule

The molecule is non-polar.

Example 3: Polar and non-polar molecules

Question

State whether methane () is polar or non-polar.

Answer

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:

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.

Example 4: Polar and non-polar molecules

Question

State whether hydrogen cyanide () is polar or non-polar.

Answer

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 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 0,4 and the electronegativity difference between carbon and nitrogen is 0,5. Both of the bonds are polar.

Determine the polarity of the molecule

The molecule is not symmetrical and so is polar.

Exercise 2: Electronegativity

In a molecule of beryllium chloride ():

  1. What is the electronegativity of beryllium?

  2. What is the electronegativity of chlorine?

  3. 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.

  4. Is the bond a non-polar or polar covalent bond?

  5. Is the molecule polar or non-polar?

  1. 2,1
  2. 3,0
  3. Hydrogen will have a slightly positive charge and chlorine will have a slightly negative charge.

    Image
  4. Polar covalent bond. The electronegativity difference is: . The bond is weakly polar.

  5. Hydrogen chloride is linear and therefore is a polar molecule.

Complete the table below:

Table 3

Molecule

Difference in electronegativity between atoms

Non-polar/polar covalent bond

Polar/non-polar molecule

     

     

     

     

     

     

     

     

     

     
Table 4

Molecule

Difference in electronegativity between atoms

Non-polar/polar covalent bond

Polar/non-polar molecule

Polar covalent bond

Polar molecule. Water has a bent or angular shape.

Polar covalent bond

Polar molecule. Hydrogen bromide is linear.

Non-polar covalent bond

Non-polar molecule.

Polar covalent bond

Non-polar molecule. Methane is tetrahedral.

Polar covalent bond

Non-polar molecule. Phosphorous pentafluoride is trigonal bypramidal and symmetrical.

Polar covalent bond

Non-polar molecule. Beryllium chloride is linear and symmetrical.

Polar covalent bond

Polar molecule. Carbon monoxide is linear, but not symmetrical.

Polar covalent bond

Non-polar molecule. Acetylene is linear and symmetrical.

Polar covalent bond

Polar molecule. Sulfur dioxide is bent or angular and is not symmetrical.

Polar covalent bond

Non-polar molecule. Boron trifluoride is trigonal pyramidal and not symmetrical.

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