We can use the differences in the electronegativity values for elements to help determine the character of a bond, that is to decide whether the bond will be ionic, covalent or polar covalent. The larger the difference in the electronegativity values between the two atoms in a bond the more unequally the electrons in that particular bond will be shared. Recall that:

• Ionic Bonding: In an idealised ionic bond, the electron is transferred from the metal atom to the non-metal atom. However in reality, this transfer of an electron completely from the metal atom to the non-metal atom rarely if ever happens and there is always some degree of electron sharing. That is there is always some degree of covalent nature to an ionic bond. However, it's important to note that the degree of covalent character in ionic compounds is relatively small, and the predominant character is of the bond is ionic.


• Covalent Bonding: In a covalent bond, the two electrons involved in forming the bond are shared equally. You might assume that covalent bonding only exists between identical atoms with identical electronegativity values, however the covalent character of a chemical bond between different non-metal elements can also be considered "purely" covalent if the electronegativity difference between the atoms forming the bond is very small.


• Polar Covalent Bonding: This type of bonding is a much more common in molecules than covalent bonding. In a polar covalent bond the electrons are shared unequally between the two atoms. The more unequal the sharing, the more polar the bond and the more ionic character it has.

You will have probably heard the phrase that pure ionic and pure covalent are the two extremes in chemical bonding and that almost all chemical bonding that occurs in molecules is to be found somewhere on the bonding spectrum between these two extremes. This is where we can use the differences in the electronegativity values to predict the character of a bond.

Bond character

We can use the differences in the electronegativity values from the table above to work out the percentage ionic/ covalent character in a bond. The table shows some simple calculations to work out the character for several common bonds found in many molecules.

From the table above you can see that bonds which we may always have considered as purely ionic such as Na-Cl do in fact have a fairly large degree of covalent character in them and bonds such as N-H which we might assume to be covalent have a fairly large amount of ionic character in them.

However we can simplify the table above to give a more user friendly table. A general rule of thumb which can help provide a good basis for determining the character a particular bond is outlined in the table below:

These rules should be used as a guide and not followed rigorously but they will help point us in the right direction as to the type of bonding present between atoms. As an example consider as an example the following three substances:

1. sodium fluoride
2. Hydrogen fluoride
3. Fluorine

Bonding present in compounds

Key Points

• Ionic and covalent bonding should be regarded as two possible extreme examples of bonding; most chemical bonds will have some degree of ionic and covalent character in them. We can use differences in the electronegativity values to make predictions about the character of a bond.
• It is important to emphasise that while most ionic compounds have some degree of covalent character, it's essential to clarify that this doesn't negate the overall ionic nature or character of the bond. Similarly some covalent compounds have ionic character but again this does not negate the overall nature or character of the covalent bond.
• The values given in the tables above should be used to inform your judgement and not assumed to be 100% accurate; they are there to guide you in deciding on the nature of any bonds present within a molecule.

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