Definition of Electronegativity
Ability of an atom to attract the bonding electrons in a covalent bond.

Electronegativity trends in the Periodic table
Whilst you will not be required to memorise the electronegativity of atoms you will have to be able to describe patterns in electronegativity in the periodic table.

· Across the periodic table electrons are added to the same shell, therefore shielding stays the same but the number of protons increases, therefore the atom has a greater ability to draw the bonding electrons in a covalent bond thus the electronegativity increases.

· Down the periodic table shielding of inner electrons increases, therefore attraction of the outer electrons decreases. Therefore the electronegativity decreases.

Elements with a higher electronegativity are more able to draw electrons towards them.

Same electronegativity
The electrons are shared equally and the bond is referred to as non-polar.

I2 Iodine I – I

Different electronegativity
The electrons are NOT shared equally, this means one atom has a greater ‘concentration’ of electrons, thus giving it a slight negative charge. The atoms with a lower electronegativity will therefore have a lower ‘concentration’ of electrons and gains a slight positive charge. This is referred to as polar and results in a permanent dipole (uneven charge distribution).

Mg – O
d+ d-

Note: d is used to represent a ‘slight’ charge

Mg has an electronegativity of 1.2, however Oxygen has an electronegativity of 3.5 and therefore the bond is polar and results in a permanent dipole.


Whilst each bond separately may be polar the overall polarity may not!

O = C = O
d- d+ d-

The element has polar bonds, however it is linear with a bond angle of 180°. It is therefore symmetrical so the charges cancel and overall is non-polar.
When looking at whether a molecule is polar always consider its shape.