4.4 A classification of chemical substances
We now have a provisional but useful classification of chemical substances. First they are divided into molecular and non-molecular types, largely on the basis of their structures. Then a further division is made according to the major source of the chemical bonding holding their atoms together. In molecular substances, the bonding is covalent, but in the non-molecular class, it may be covalent, ionic or metallic. This classification is shown in Figure 32. For a recent and interesting example of a substance changing categories within this classification, see Box 4.
Box 4: Turning dry ice into sand
As Mendeléev emphasised, the highest valencies of the elements are the clearest chemical sign of periodicity. At first glance, the highest oxides of carbon and silicon are quite different. CO2 is a gas, which freezes to a molecular solid at −79 °C; SiO2 is a non-molecular solid melting at over 1 500 °C. But despite these differences, both are dioxides. In both compounds, carbon and silicon exercise a valency of four, and this is why Mendeléev put both elements in the same Group.
Even the differences are not unalterable. In solid carbon dioxide (Figure 7), the distances between molecules are relatively large. The quartz structure of SiO2 (Figure 11) is non-molecular, with identical short distances between neighbouring atoms. It is therefore more compact. When pressure is applied to a solid, it encourages a change into more compact forms. So at high pressures, solid CO2 might shift to a silica-like structure. Raising the temperature should also help by speeding up any change.
In 1999, scientists at the Lawrence Livermore laboratory in California subjected solid carbon dioxide to 400 kilobars pressure. This is 400 times the pressure at the bottom of the Mariana Trench, the deepest point in the world's oceans. At these pressures, the CO2 stayed solid even when the temperature was raised to 2000 °C. The Livermore scientists then used the following technique: they determined the Raman spectrum of the solid, a type of vibrational spectrum. It showed (Figure 33) that under these conditions the carbon dioxide had assumed a silica-like structure. Dry ice had taken on the structure of sand!