Electronegativity and polarisation  

​​​​In a nutshell

Electronegativity is the ability for an atom to attract a bonded pair of electrons in a covalent bond. The electronegativity of different elements are represented by the Pauling scale. 

Electronegativity 

​​Definition 

Electronegativity is the ability for an atom to attract a bonded pair of electrons in a covalent bond.

Electronegativity is measured using the Pauling scale. A higher value on the scale represents an element being more electronegative. More electronegative elements will have a higher nuclear charge, with more protons in the nucleus leaving a smaller radii.  Electronegativity will increase across a period and decrease down a group. 

Example 

The electronegativity decreases down Group $$7$$​. Therefore, the values given according to the Pauling scale will decrease down the group. 

Polarity

The polarity of a covalent bond will depend on the electronegativity of each atom present.  

A covalent bond is polar when formed between atoms with different electronegativities. The shared electrons will be spread unevenly, as one atom is more positive than the other. The shift in electron density, due to the difference in electronegativity, will form a dipole. A dipole is a charge difference between two atoms.   

Example 

Hydrogen bromide contains a hydrogen and bromine atom. Both of these atoms have different electronegativities so a polar bond will form between them.

The hydrogen atom has a slightly positive charge ($$\delta+$$), whereas the chlorine atom has a slightly negative charge ($$\delta-$$). Due to the slight negative charge on the chlorine atom, electrons are dragged towards it. 

When two atoms have the same electronegativity, the covalent bond will be non-polar. This can occur when the same type of atom are bonded together, or when two atoms have a very similar electronegativity. 

Example 1

Hydrogen is a diatomic gas. As this molecule contains two of the same atoms bonded together, a non-polar covalent bond will form. Both hydrogen atoms have the same electronegativities.  

Example 2

Carbon and hydrogen have very similar electronegativity values, so therefore a non-polar bond is formed for $$C-H$$ bonds. 

Percentage ionic character

Electronegativity and Pauling values can be used to work out which type of bond is most likely to form. The larger the difference in electronegativity values for two atoms, the more likely it is for an ionic bond to form. 

procedure 

Example 

Given that the Pauling electronegativity values for carbon is $$2.5$$​ and bromine $$3.0$$​, find the percentage ionic character for this bond. 

First, work out the difference between the electronegativity of the carbon and bromine atom:

$$3\,-\,2.5\,=\,0.5$$​​

Look in the table at the electronegativity difference to find the percentage ionic character:

The percentage ionic character for a $$C-Br$$ bond is $$\underline{6\%}$$. 

Molecule polarity 

Although a molecule contains polar bonds, it might not be polar. If polar bonds are arranged so they point in opposite directions, the polar bonds will cancel each other out leaving the molecule non-polar. However, if all of the polar bonds point in the same direction the molecule will be polar. 

Example

​$$BF_3$$ has three polar $$B-F$$ bonds. However, as each three bonds point in opposite directions the molecule is non-polar overall.    

There are many examples of polar and non-polar molecules. Below is a table with some examples.