1-5 Bonding In Compounds (part 3)

V. Bonding in compounds

Part 3

Topic

V.

Bonding in compounds

Reference Reading

Integrated Chemistry Today (2nd Ed.), L.H.M Chung, Book 1A, 93

Objectives

1.5.1.4 – recognise the presence of weak intermolecular forces called van der Waals forces among molecules.

Notes

5.

Page 1

Part 3

Van der Waals' forces (intermolecular forces)

a) Polar and non-polar molecules (Details are not required) Molecules can be classified into polar molecules and non-polar molecules. A molecule is called polar if a positive pole and a negative pole can be found on the molecule. Otherwise, it is a non-polar one.

δ is pronounced as ‘delta’ which means a little. The two poles only carry only a little charges comparing with the isolated + and - charges on the ions. Since a polar molecule possesses a positive pole and a negative pole, it is also called a dipole.

b) Origin of polarity of a molecule The polarity of a molecule is caused by the uneven distribution of the electrons in a molecule. Water is a polar molecule because the electronegativity of oxygen is higher than that of hydrogen.

Oxygen atom draws the electrons inside the water molecule towards itself. This makes the region close to the oxygen atom relatively rich in electron and carries a little negative charge. Another end of the molecule (the region close to hydrogen atom) will become poor in electron and carries a little positive charge. N.B.

As a whole, the whole molecule is still electrically neutral.

Although water consists of molecules, it is a liquid instead of a gas at room temperature. This means that the molecules are not totally free from the attractions of the others. The attractive forces acting among separate molecules are called intermolecular forces. Because it was first discovered by the scientist van der Waals, intermolecular forces is also called van der Waals' forces.

V. Bonding in compounds

Part 3

Page 2

Intermolecular forces (van der Waals' forces) Comparatively, the strength of intermolecular forces is only

1 of those of covalent bond. 100

Moreover, the strength of intermolecular forces among different molecules are also different.

Obviously, the intermolecular forces among polar molecules is stronger than the intermolecular forces among non-polar molecules.

Intermolecular forces among polar molecules (relatively stronger) e.g. H2O (m.p. 0 ºC, b.p. 100 ºC)

Note :

Glossary

Past Paper Questions

Intermolecular forces among nonpolar molecules (relatively weaker) e.g. N2 (m.p. -210 ºC, b.p. -196 ºC )

Because even non-polar substance liquefies and freezes at low temperature, intermolecular forces also exist among non-polar molecules.

polar molecule non-polar molecule forces van der Waals' forces dipole

delta

polarity

electronegativity

intermolecular