Chemistry, Third edition

FORCES BETWEEN COVALENT MOLECULES

Forces between covalent molecules

So far, we have discussed the bonding within covalent molecules (intramolecular

bonding). In this section, we will describe the forces that hold covalent molecules

(and some neutral atoms) together in the liquid and solid states (intermolecular

bonding). Intermolecular forces have the general name van der Waals’ forces, after

the Dutch scientist Johannes van der Waals (1837–1923). The different types of

intermolecular forces discussed are shown in Table 5.2.

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Table 5.2Types of intermolecular forces

Types of molecules Types of intermolecular

forces between molecules

Molecules with dipoles (i) dipole–dipole attractions

(ii) hydrogen bonding

(iii) London dispersion forces

Molecules (or atoms) without dipoles London dispersion forces only

Dipole–dipole interactions

Molecules which have dipoles, X–Y, tend to be attracted towards one another

as shown in Fig. 5.11. The negative end of one molecule is attracted to the positive

end of another and vice versa. Because these attractions are between partialcharges

(and), they are much weaker electrostatic interactions than exist between

Another allotrope of carbon?

Chemists in the USA have reported the discovery of yet another allotrope of carbon, called

LAC. LAC consists of long chains of carbon atoms, where the alternate carbon–carbon bonds

are of different lengths. Remembering that carbon atoms have four valence electrons, can

you suggest a structure for LAC? Hint: LAC stands for linear acetylenic carbon.

Exercise 5I

Fig. 5.11The attraction between molecules with dipoles.

Smalley and Robert Curl – were jointly awarded the 1996 Nobel Prize for Chemistry

for the discovery of C 60.