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.





Hydrogen (\(\text{H}\))


Lithium (\(\text{Li}\))


Beryllium (\(\text{Be}\))


Boron (\(\text{B}\))


Carbon (\(\text{C}\))


Nitrogen (\(\text{N}\))


Oxygen (\(\text{O}\))


Fluorine (\(\text{F}\))


Sodium (\(\text{Na}\))


Magnesium (\(\text{Mg}\))


Aluminium (\(\text{Al}\))


Silicon (\(\text{Si}\))


Phosphorous (\(\text{P}\))


Sulfur (\(\text{S}\))


Chlorine (\(\text{Cl}\))


Potassium (\(\text{K}\))


Calcium (\(\text{Ca}\))


Bromine (\(\text{Br}\))


Table: Table of electronegativities for selected elements.

Definition: 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.

Example: Calculating Electronegativity Differences


Calculate the electronegativity difference between hydrogen and oxygen.

Step 1: 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 \(\text{3.5}\).

Step 2: Calculate the electronegativity difference

\(\text{3.5} – \text{2.1} = \text{1.4}\)

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