When a covalent bond forms between two atoms of the same element, e.g. O2, the bonding electrons are shared equally between the two atoms. This means each electron spends equal time moving around each atom.
When covalent bonds form between two different atoms, electrons are normally not equally shared since they have different electronegativity. Thus, the electrons may spend more time at one atom than the other, since it attracts electrons more. The region near the more electronegative atom has a small negative charge, and the region near the less electronegative ion has a small positive charge.
Extreme Polar Bonds
When two atoms have a very large difference in electronegativity, an ionic bond is formed. Effectively, the electron spends all of its time at the more electronegative atom, forming an ion.
Polarity of Molecules
A polar molecule is one which has a net uneven charge distribution. A molecule is polar (or called an electric dipole) when:
- It contains polar covalent bonds; bonds between atoms of different electronegativity
- It is asymmetrical – so the centres of partial positive and negative charges do not correspond and the vectors do not cancel out.
In the above figure, HCl contains polar bonds, but is not symmetrical. Hence it is a polar molecule. Methane contains polar bonds, but is symmetrical. Therefore, it is a non-polar molecule.