Electronegativity

• Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond.

• Measured using the Pauling scale.

• Fluorine is the most electronegative element and is given the value 4.0.

• Electronegativity increases across period and decreases down groups (excluding the noble gases).

Polar bonds

• When 2 atoms that are covalently bonded have the same or similar electronegativities, they attract the bonding electrons equally and so share the electrons equally.

• This results in a non-polar bond.

• E.g. H-H, Cl-Cl, C-C, H-C

• When 2 atoms that are covalently bonded have different electronegativities, the more electronegative atom attracts the bonding electrons more, so the bonding electrons are unevenly shared between the atoms.

• The more electronegative atom will have a partial negative (δ-) charge while the other atom will have a partial positive (δ+) charge. Therefore the bond is said to be polar.

• E.g. H-O, H-N, C=O, C-Cl

• In a polar bond the difference in electronegativity between the 2 atoms causes a dipole.

• A dipole is a difference in charge between the 2 atoms caused by a shift in electron density.

• A polar bond can be denoted using an arrow pointing towards the more electronegative atom.

• The Pauling scale can be used to work out whether a bond is polar or not.

• Usually a bond is polar if the difference in electronegativity between the 2 atoms is greater than 0.4.

Polar molecules

• Polar molecules have an overall dipole

• A molecule can contain polar bonds and not be a polar molecule (e.g. CO₂)

• This is because if the molecule is symmetrical and the dipoles point in opposite directions then they will cancel out.

• If the dipole point in the same direction the molecule will be polar.