Intermediate bonding and bond polarity

Ionic to covalent is a continuum.

COVALENT

When the two atoms bonded are the same, the the nuclei attraction is the same, so the pair of electrons is shared equally.

When the two atoms are different, the attraction by the nuclei may be different, so the pair of electrons is not equally shared.

Electronegativity is a measure of the attraction of an atom in a molecule for a pair of electrons in a covalent bond. 

This is used to measure electronegativity.

It is a relative scale and has no units. It runs from 0 to 4. The higher the number, the higher the electronegativity. 

You can calculate to what degree a bond is covalent and to what degree is it ionic. 

When both atoms in the molecule are the same, and the electrons are equally shared, we can say the centre of charges coincide. 

However, when they are not the same one atoms pulls the electron pair closer and this distorts the electron cloud. 

This distortion results in the molecule having a small charge. 

In a similar way ionic bonds can be distorted by the attraction of the cation for the outer anion.

If this distortion is great it can lead to a charge cloud, which resembles a covalent bond.

The more electronegative the atom, the larger the attraction of the electron pair resulting in