Philippa B. Cranwell

Foundations of Chemistry


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dipole showing charge distribution; (c) neighbouring chlorine molecule with an induced dipole, showing charge distribution.

      This is because the shift of electrons generating a δ− charge forces the electrons in a nearby bond to be repelled, so a dipole is formed in the nearby molecule, as shown in Figure 2.30c. London dispersion forces are reasonably weak because they are short‐lived, but they are important nevertheless.

      Instantaneous dipoles (or dispersion forces) increase with increasing polarisability of the molecule. The more readily polarisable the molecule, the larger the instantaneous dipole and induced dipole. Polarisability is a measure of how easily the charge distribution in an atom or molecule can be distorted by the application of an external electrical field or charge. The greater the number of electrons, the more readily polarisable the molecule is, so instantaneous dipoles increase with increasing molecular mass. In addition, the larger the surface area of a molecule or the larger the area of possible contact between two molecules, the stronger the intermolecular forces.

Schematic illustrations of (a) Helium atom showing even distribution of electrons; (b) helium atom with instantaneous dipole due to temporary movement of charge; (c) induced dipole in a neighbouring helium atom.

      A monatomic molecule is composed of just one atom. A diatomic molecule such as Cl2 has two atoms. A triatomic molecule such as H2O has three atoms. A polyatomic atom has several atoms.

      2.4.3 Hydrogen bonding

      The final type of interaction is hydrogen bonding, which is the strongest type of intermolecular force and has about one‐fifth the strength of a typical covalent bond. Hydrogen bonding is actually a special type of dipole–dipole interaction. Hydrogen bonds are formed between molecules that contain a hydrogen atom bonded to a small, strongly electronegative element such as nitrogen, oxygen, or fluorine.

      Hydrogen bonding can occur when a hydrogen atom in a molecule is bonded to a strongly electronegative element such as nitrogen, oxygen, or fluorine. A hydrogen bond is formed between the H atom of one molecule and the N, O, or F atom of a neighbouring molecule.

Schematic illustrations of (a) the formation of a hydrogen bond between two molecules of water where molecules are in constant motion; (b) formation of hydrogen bonds in ice, where molecules are in a fixed position.

      In liquid water, hydrogen bonds are constantly forming and breaking, as in Figure 2.32a, whereas in ice (Figure 2.32b), the molecules are held together in fixed positions by hydrogen bonds. Each oxygen atom can form two hydrogen bonds to neighbouring molecules as it possesses two lone pairs of electrons.

Schematic illustrations of (a) hydrogen bonding between two ethanol, C2H5OH, molecules; (b) hydrogen bonding between ethanol, C2H5OH, and water molecules.

      2.4.4 Summary of strengths of intermolecular forces

      Intermolecular forces are the weak forces of attraction formed between simple covalently bonded molecules. There are three types of intermolecular forces:

       London dispersion forces: instantaneous dipole–induced dipole forces

       Permanent dipole–permanent dipole forces

       Hydrogen bonding

      The order of strengths of these forces is:



Bond type/intermolecular force type Strength (approx.)/kJ mol−1
Ionic bond (e.g. NaCl) 771
Single covalent bond (e.g. Cl—Cl) 242