Читать книгу Geochemistry - William M. White - Страница 20

Except for the noble gases, atoms rarely exist independently; they are generally bound to other atoms in molecules, crystals, or ionic radicals. Atoms bind to one another through transfer or sharing of electrons, or through electrostatic forces arising from uneven distribution of charge in atoms and molecules. A bond that results from the transfer of electrons from one atom to another is known as an ionic bond, an example is the bond between Na and Cl in a halite crystal. In this case, the Na atom (the electropositive element) gives up an electron, becoming positively charged, to the Cl atom (the electronegative element), which becomes negatively charged. Electrostatic forces between the Na⁺ and the Cl^– ions hold the ions in place in the crystal. When electrons are shared between atoms, such as in the H₂O or CH₄ molecules or the radical, the bond is known as covalent. In a covalent bond, the outer electrons of the atoms involved are in hybrid orbits that encompass both atoms.

Ideal covalent and ionic bonds represent the extremes of a spectrum: most bonds are neither wholly covalent nor wholly ionic. In these intermediate cases, the bonding electrons will spend most, but not all, of their time associated with one atom or another. Electronegativity is useful in describing the degree of ionicity of a bond: a bond is considered ionic when the difference in the electronegativity of the two atoms involved is greater than 2. In Figure 1.5, we see that metals tend to have low electronegativities while the nonmetals have high electronegativities. Thus, bonds between metals and nonmetals (e.g., NaCl) will be ionic while those between nonmetals (e.g., CO₂) will be covalent, as will bonds between two like atoms (e.g., O₂).

Another type of bond occurs in pure metal and metal alloy solids. In the metallic bond, valence electrons are not associated with any single atom or pair of atoms; rather, they are mobile and may be found at any site in the crystal lattice. Since metals rarely occur naturally at the surface of the Earth (they do occur in meteorites and the Earth's core), this type of bond is less important in geochemistry than other bonds.

Ionically bonded compounds tend to be hard, brittle, and highly soluble in water. Covalently bonded compounds tend to be good conductors of heat, but not of electricity. They are typically harder and less brittle than ionic solids but less soluble. In molecular solids, such as ice, atoms within the molecule are covalently bonded. The molecules themselves, which occupy the lattice points of the crystal, are bonded to each other through van der Waals and/or hydrogen bonds. Such solids are comparatively weak and soft and generally have low melting points.

Molecules in which electrons are unequally shared have an asymmetric distribution of charge and are termed polar. A good example is the hydrogen chloride molecule. The difference in electronegativity between hydrogen and chlorine is 0.9, so we can predict that the bonding electron will be shared but associated more with the Cl atom than with the H atom in HCl. Thus, the H atom will have a partial positive charge, and the Cl atom a partial negative charge. Such a molecule is said to be a dipole. The dipole moment, which is the product of one of the charges (the two charges are equal and opposite) times the distance between the charges, is a measure of the asymmetric distribution of charge. Dipole moment is usually expressed in Debye units (1 Debye = 3.3356 × 10^–34 coulomb-meters).