Читать книгу Earth Materials - John O'Brien - Страница 52

Because the distributions of electrons in the electron cloud are probabilistic and constantly changing, they may be, at any moment, asymmetrically distributed within the electron cloud. This asymmetry gives rise to weak electric dipoles on the surface of the electron cloud; areas of excess negative charge concentration where the electrons are located and areas of negative charge deficit (momentary positive charge) where they are absent. Areas of momentary positive charge on one atom attract electrons in an adjacent atom, thus inducing a dipole in that atom. The areas of excess negative charge on one atom are attracted to the areas of positive charge on an adjacent atom to form a very weak bond that holds the atoms together (Figure 2.17). Bonds that result from weak electric dipole forces that are caused by the asymmetrical distribution of electrons in the electron cloud are called van der Waals bonds. The presence of very weak van der Waals bonds helps to explain why minerals such as graphite and talc are extremely soft and have a “greasy” feel (Chapter 5).

Figure 2.17 Van der Waals bonding occurs when one atom becomes dipolar as the result of the random concentration of electrons in one region of an atom. The positively charged region of the atom attracts electrons in an adjacent atom causing it to become dipolar. Oppositely charged portions of adjacent dipolar atoms are attracted creating a weak van der Waals bond. Larger structures result from multiple bonds.

Figure 2.18 Diagram showing two water molecules joined by a hydrogen bond that links the hydrogen in one molecule to the oxygen in the other molecule.

Hydrogen bonds exist between electropositive hydrogen and electronegative ions such as oxygen in molecules such as water or hydroxyl ions. Because of the profound importance of water (H₂O) and hydroxyl ion (OH⁻¹), in both organic and inorganic compounds, this type of bond has been given its own separate designation (Figure 2.18). Hydrogen bonds are relatively weak bonds that occur in hydrated (water‐bearing) or hydroxide (hydroxyl‐bearing) minerals.

Atoms are held together by a variety of chemical bonds. The type of bond that forms depends largely on the electron configurations of the combining elements, as expressed by their electronegativities, although environmental factors also play a role. Each bond type imparts certain sets of properties to Earth materials that contain those bonds. In the following section we will discuss factors that determine the three‐dimensional properties of the molecular units that result from such bonding. In Chapter 4 we will elaborate on the long‐range crystalline structures that form when these molecular units combine to produce crystals. Remember: it all starts with atoms, their electron properties and the way they bond together to produce crystals.