Electronegativity and polarity

What is a pure covalent bond?

The shared pair of electrons are equally attracted to the nuclei of two identical atoms in the bond (e.g. H-H)

Definition of electronegativity

A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond

Trend in electronegativity

Increases up and across the periodic table

• Nuclear charge increases

• Atomic radius decreases

• Decreased shielding

The stronger the attraction between the nucleus and the bonding pair of electrons - the greater electronegativity for that atom

Non-polar covalent bonds

The bonding pair of electrons is shared equally between the bonded atoms due to:

• the bonded atoms being identical

• the bonded atoms having the same or similar electronegativity values

Polar covalent bonds

The bonding pair of electrons is shared unequally between the bonded atoms

Permanent dipole

A small charge difference across a bond that results from a difference in electronegativities of the bonded atoms

• Polar covalent bonds have permanent dipoles

What determines whether a molecule is polar

• Shape of the molecule 

• Polarity of it’s bonds 

• A molecule can be non-polar even if it contains polar bonds, due to the molecules shape

• Symmetrical molecule = dipoles cancel out