Читать книгу Geochemistry - William M. White - Страница 22

Molecules, which result from the chemical bonds between atoms discussed above, are a familiar concept. By some definitions, they are electrically neutral; charged species consisting of two or more atoms are known as radicals. The properties of molecules are generally quite different from those of their constituent atoms. Carbon dioxide is a good example: the equilibrium form of pure carbon at the room temperature and pressure is a solid, graphite, while that of oxygen is a diatomic gas. The properties of molecules depend on the bond lengths, which depend on the strength of the chemical bond, as well as the geometric arrangement of the atoms. For example, the polar nature of the water molecule, from which many of its unusual properties arise (and which will discuss in Chapter 3), including the formation of hydrogen bonds, is a consequence of the tendency of valence electron pairs surrounding an atom tend to repel each other which results in an arrangement in which the two hydrogens are separated by an angle of 104.45° (Figure 1.8a) and a partial positive charge on the hydrogens and a partial negative one on the other side of the molecule In contrast, the arrangement of atoms in CO₂ is linear with a bond angle of 180°. CO₂ reacts with water to produce carbonic acid (H₂CO₃), which has a plane trigonal geometry.

Figure 1.8 (a) Geometry of the water molecule. (b) Hydrogen bonds between water molecules. The δ+ and δ– indicate partial positive and negative charges, respectively.

Geometry becomes enormously important for organic molecules and life. For example, C₁₂H₂₂O₁₁ is the chemical formula for both lactose and sucrose, as well as several other disaccharide carbohydrates, but the atoms are stitched together differently and as a result they have quite different properties. Among other things, all adults (and essentially all animals) can readily digest sucrose, but many adult humans (and most adult mammals) cannot digest lactose. In other molecules, even slight variations in structure, for example, a molecular structure and its mirror image can have quite different properties – a topic we'll explore briefly in Chapter 12.

Molecules are not necessarily static entities. An important feature of some molecules is the ability to dissociate. This is particularly true of both water and carbonic acid, which can give up hydrogen atoms. Acidity reflects the balance between H⁺ (strictly speaking H₃O⁺) and OH^– ions; these must be equal in pure water, but a solution of CO₂ in water will have an excess of H⁺ and hence be acidic. These hydrogen ions can also reassociate with their parent molecules and do so when H ions become abundant.

The bonds between atoms in molecules are also not static, but rather bond lengths and bond angles continually oscillate about their mean values. For example, the water molecule has three fundamental modes of vibration: two stretching vibrations of the O-H bond (one symmetric, one asymmetric) and a bending vibration in which the bond angle changes. Vibrational frequencies are proportional to bond strength and increase with increasing temperature (in stepwise fashion, as they are quantized), although many molecules remain in the ground state vibrational frequencies over the range of temperatures at the Earth's surface. These vibrations are responsible for some of the interaction of molecules with light. For example, the frequencies of the stretching vibrations of water and CO₂ correspond to near-infrared electromagnetic frequencies and as a consequence both molecules absorb infrared photons, and hence both are important greenhouse gases.