1.4 Molecular substances
Chlorine, bromine and iodine belong to a family of elements called the halogens. At room temperature, chlorine (Figure 2e) is a gas, bromine (Figure 2d) is a liquid and iodine is a dark-purple solid. All three substances are chemical elements. One's first thought might be that the tiny particles of which, say, chlorine gas is composed are single atoms.
Is this the case?
No; the tiny particles or molecules consist of pairs of chlorine atoms, Cl2.
A gas, like chlorine, occupies much more space than a solid or liquid, so the distance between the molecules is comparatively large. At normal temperatures and pressures, it averages about 3 500 pm (1 pm ≡ 10−12m), compared with a distance of only 198 pm separating the chlorine atoms in gaseous Cl2 molecules (Figure 6a). This disparity is less extreme, but still evident in liquid bromine and solid iodine. The positions of atoms in solids can be determined by X-ray crystallography. In solid iodine (Figure 6b), each iodine atom has a second iodine atom at a distance of only 271 pm. By contrast, in other directions, the shortest distance to another iodine atom is considerably greater (350 pm). So the iodine atoms can be grouped into pairs; hence we conclude that solid iodine contains I2 molecules.
Similar reasoning can be used to identify molecules in compounds. At room temperature, carbon dioxide is a gas containing CO2 molecules. On cooling, it becomes a solid ('dry ice'). In dry ice (Figure 7), each carbon atom, A, has two oxygen atom neighbours, B and C, at a distance of 116 pm. These three atoms are colinear. The next nearest atom is another oxygen, D, at 311 pm. Here is evidence that solid carbon dioxide contains linear CO2 molecules, with the atom sequence O—C—O.
The formulae Cl2, Br2, I2 and CO2 that we have identified for the three halogens and carbon dioxide are called molecular formulae. They tell us how the atoms are grouped together in the molecules from which the substance is built up. Likewise, the four substances are called molecular substances because they have structures that allow discrete molecules to be picked out. So far, we have examined just one molecular compound (CO2) and its molecular formula is identical with its empirical formula, but often this is not so. In Section 1.2, we discussed solid aluminium bromide with empirical formula AlBr3. Here, the molecular and empirical formulae are not identical: the crystal structure contains Al2Br6 molecules (Figure 8).
Do these molecules have the same empirical formula as the solid in which they are found?
Yes; the molecular formula is Al2Br6, but in both the molecules and the solid, the ratio of aluminium atoms to bromine atoms is 1 : 3. In molecular substances that contain just one type of molecule, that molecule has the same empirical formula as the compound.
The so-called organic compounds formed by the element carbon are almost entirely molecular. To mark this point, we show, in Figure 9, the grouping of the atoms in the molecules of two important solid organic compounds. Figure 9a shows the structure of aspirin, the best-known painkiller, which is also used in the precautionary treatment of heart conditions. The molecule in Figure 9b is RDX, the most common military high explosive. Here, you need not worry about the names used for organic compounds. In this course, relatively few such compounds are discussed, and we shall be concerned only with differences in the structure of their molecules; the names are just labels.