Chemistry, Third edition

FORCES BETWEEN COVALENT MOLECULES 77

Hydrogen bonding

Hydrogen bonding is a special type of dipole–dipole attraction in which the hydrogen

atom acts as a bridge between two electronegative atoms. A hydrogen bond follows the

general formula

A——H· · · · · B

Examples of hydrogen bonding are F–H··· O, where A is F and B is O, and

F–H··· F, where A and B are F (see Example 5.6). These three atoms are

usually bonded in a straight line. A and B are the electronegative atoms, such as F, O

or N. Such atoms possess one or more lone pairs of electrons.

BOX 5.5

London dispersion forces and the shapes of molecules

Molecules of pentane (C 5 H 12 ) exist with the

same number of electrons, but very different

shapes. One form of C 5 H 12 has molecules

which have a cylindrical shape, whereas the

other has molecules which are spherical. The

cylindrical form has a boiling point of 36 °C,

while the spherical form boils at 10 °C,

suggesting that weaker London interactions

exist between the spherical molecules. The

valence electrons in the molecules with a

spherical shape are ‘hidden’ deeper within

the molecule than if the molecule is

cylindrical, and cannot interact as strongly

with neighbouring molecules (Fig. 5.14).

Fig. 5.14Different shapes of pentane

molecules.

Example 5.6

A molecule of hydrogen fluoride has a large dipole moment. The partial positive

charge on the hydrogen is attracted to the lone pair of electrons on the partially

negatively charged fluorine atom of a neighbouring molecule – it forms a hydrogen

bondwith the fluorine. Liquid hydrogen fluoride contains zigzag chains of

molecules of HF joined together by hydrogen bonds (Fig. 5.15).

Fig. 5.15Hydrogen bonding between molecules of

hydrogen fluoride (hydrogen bonds are usually represented

by dotted lines).

London forces

(i)Arrange the following in order of decreasing

boiling points. Explain your answer.

Inert gas Diameter of atom/pm

He 100

Ne 130

Ar 190

Kr 220

(ii)At room temperature, F 2 and Cl 2 are

gases, whereas Br 2 is a liquid and I 2 a solid.

Explain these observations in terms of the

strength of intermolecular forces within

these substances.

Exercise 5J