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 between the two atoms in a bond the more unequally the electrons in that bond will be shared. In an ionic bond the electron is completely transferred from the metal atom to the non-metal and in a covalent bond the 2 electrons involved in forming the bond are shared equally. However ionic and covalent bonding should be viewed as two extreme examples of bonding types. A much more likely scenario is one where the electrons are shared unequally between the two atoms involved in forming a polar covalent bond. The more unequal the electrons involved in bonding are shared the more polar the bond will be and the more ionic character the bond will have. The more equally the electrons are shared the more covalent character will be in the bond. This is where we can use the differences in the electronegativity values to predict the character of a bond. The table below shows how the amount of ionic and covalent character in a bond due to differences in electronegativity values between the two elements involved in forming the bond.
| Difference in electronegativity | % ionic character in bond | % covalent character in bond |
|---|---|---|
| 0.1 | 0.5 | 99.5 |
| 0.3 | 2 | 98 |
| 0.5 | 5 | 95 |
| 0.7 | 11 | 89 |
| 0.9 | 20 | 80 |
| 1.1 | 25 | 75 |
| 1.3 | 35 | 65 |
| 1.5 | 43 | 57 |
| 1.7 | 50 | 50 |
| 1.9 | 60 | 40 |
| 2.1 | 68 | 32 |
| 2.3 | 75 | 25 |
| 2.5 | 80 | 20 |
| 2.7 | 85 | 15 |
| 2.9 | 89 | 11 |
| 3.1 | 92 | 8 |
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 below has done this for several common bonds found in many compounds.
| Bond and electronegativity value | Difference in electronegativity between the atoms | % ionic character/covalent character |
|---|---|---|
| C-H 2.5 2.1 |
0.4 | 4% ionic/96% covalent |
| C-O 2.5 3.5 |
1.0 | 22% ionic/78% covalent |
| N-H 3.0 2.1 |
0.9 | 20% ionic/80% covalent |
| Na-Cl 0.9 3.0 |
2.1 | 68% ionic/32% covalent |
| K-F 0.8 4.0 |
3.2 | 92% ionic/8% covalent |
From the table above you can see that bonds which we may always have considered as purely ionic such as Na-Cl 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 and working table. A general rule of thumb which can help provide a good basis for determining the type of bonding present in a particular bond is shown in the table below:
| Ionic | Polar covalent | Covalent |
|---|---|---|
| Bonds between atoms with large differences in electronegativity of 1.9 or more will have ionic bonding between them. | Bonds between atoms with electronegativity differences between 0.5-1.9 will be polar covalent. | Bonds between non-metal elements with small differences in electronegativity values of around 0.5 and below will be covalent. |
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:
| Element | Na | H | F |
|---|---|---|---|
| Electronegativity | 0.9 | 2.1 | 4.0 |
| Compound | Difference in electronegativity | Bonding present |
|---|---|---|
| sodium fluoride | 4.0 - 0.9 = 3.1 | Ionic bonding. There is a very large difference in the electronegativity (3.1), well above the threshold of 1.9 for ionic bonding. |
| hydrogen fluoride | 4.0 - 2.1 = 1.9 | Polar covalent bonding present. The electrons in the H-F bond will be most located towards the fluorine atom due to its large electronegativity value |
| fluorine | 4.0 - 4.0 = 0 | No difference in electronegativity so bonding will be covalent |