Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 125

An instantaneous dipole can occur in a bond between any two elements, regardless of the electronegativities of the bonded atoms. As the name suggests, they are fleeting and so do not last very long, but they can have an impact upon other molecules that are nearby. The common name for an instantaneous dipole to induced dipole interaction is London dispersion forces.

If the atoms in a bond have similar electronegativities, the electron charge is evenly distributed between them. However, because electrons are constantly moving, there is still a chance that at any one moment, the electrons may suddenly be at one end of the bond, rendering that end of the bond slightly negatively charged (δ−) and the other end of the bond slightly positively charged (δ+) in comparison. This forms an instantaneous dipole. Once an instantaneous dipole has been set up, it induces a dipole in another bond in a nearby molecule: Figure 2.30.

Figure 2.30 (a) Chlorine molecule with even distribution of charge; (b) Chlorine molecule with instantaneous 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.

Instantaneous dipoles are responsible for the very weak intermolecular forces formed between noble gas atoms. The noble gases consist of monatomic molecules: single atoms of neon, argon, etc. Clearly, there are no intramolecular forces here, as there are no bonds, and the intermolecular forces are extremely weak. Instantaneous dipoles are formed by the random movement of electron density from one side of the atom to another, which then induces a dipole in a neighbouring atom, as shown in Figure 2.31. Helium has only two electrons, so the size of the dipole is very small. The heavier noble gases possess more electrons and so have larger dipoles.

Figure 2.31 (a) Helium atom (Z = 2) 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.