2.2.2 Electronegativity and Bond Polarity

What is electronegativity?

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

How are Pauling electronegativity values interpreted?

Higher Pauling values mean a stronger ability to attract bonding electrons;

lower values mean weaker attraction.

Which elements are the most electronegative?

F, O, N and Cl.

How does electronegativity change across a period?

It increases because the number of protons increases and atomic radius decreases (electrons in the same shell are pulled in more).

How does electronegativity change down a group?

It decreases because the distance between the nucleus and outer electrons increases and shielding by inner shells increases.

What type of bond forms between atoms with similar electronegativity?

A purely covalent bond.

What type of bond forms between atoms with different electronegativities?

A polar covalent bond.

What is a polar covalent bond?

A bond with an unequal distribution of electrons, creating a charge separation (dipole) with δ⁺ and δ⁻ ends.

In a polar covalent bond, which atom becomes δ⁻?

The atom with the higher electronegativity.

Example of a polar bond?

H–Cl, where Hδ⁺ and Clδ⁻.

What is a permanent dipole?

A separation of charge within a bond or molecule

due to unequal electron distribution (δ⁺ and δ⁻ ends).

What is meant by a polar molecule?

A molecule with an overall dipole moment due to the presence of permanent dipoles that do not cancel out.

When is a molecule non-polar despite containing polar bonds?

When the molecule is

• symmetrical

• identical bonds

• no lone pairs

so dipoles cancel out.

Example of a non-polar molecule.

CO₂ or CCl₄ — both symmetrical, so dipoles cancel.

Example of a polar molecule.

CH₃Cl — asymmetrical, so dipoles do not cancel.

How does molecular shape affect polarity?

If the dipoles in the molecule add vectorially to give a net dipole, the molecule is polar

if they cancel, it is non-polar.

What type of bond forms when the electronegativity difference is very large?

An ionic bond.

What determines the type of bonding between atoms?

The difference in electronegativity between them.