Intermolecular Forces 2

In the previous chapter, we discussed the different forces that exist between atoms (intramolecular

forces). When atoms arejoined to one another they form molecules, andthese molecules in turnhave

forces that bind them together. These forces areknown as intermolecular forces, and we are going to

look at them in more detail in this next section.

See introductory video:VPhgb at http://www.everythingscience.co.za

DEFINITION: Intermolecular forces

Intermolecular forces are forces that act betweenstable molecules.

You will also recall fromthe previous chapter, that we can describe molecules as being either polar

or non-polar. A polar molecule is one in which there is a difference in electronegativity between the

atoms in the molecule,such that the shared electron pair spends moretime close to the atomthat

attracts it more strongly.The result is that one end of the molecule will have a slightly positive charge

(δ+), and the other end will have a slightly negativecharge (δ+). The molecule is saidto be a dipole.

However, it is important to remember that justbecause the bonds within a molecule are polar, the

molecule itself may notnecessarily be polar. The shape of the moleculemay also affect its polarity. A

few examples are shownin Table 2.1 to refresh your memory!

Table 2.1: Polarity in molecules with different atomic bonds and molecular shapes

Molecule Chemical

formula

Bond

between

atoms

Shape of molecule Polarity of

molecule

Hydrogen H 2 Covalent Linear molecule

H H

Non-polar

Hydrogen chlo-

ride

HCl Polar cova-

lent

Linear molecule

Hδ

+

Clδ

−

Polar

Carbon tetrafluo-

ride

CF 4 Polar cova-

lent

Tetrahedral molecule

Cδ

+

Fδ

−

Fδ

−

Fδ−

Fδ

−

Non-polar