Alkali halides

Type of physical science: Chemistry

Field of study: Chemical compounds

Alkali halides are the family of compounds formed by the combination of one of the six alkali metal elements with one of the five halogens. They are hard, transparent, insulating ionic solids with high melting points that dissolve readily in water and other polar solvents to form electrically conducting solutions.

Overview

The alkali halides are among the simplest and most abundant chemical compounds known. These compounds are formed by the combination of any one of the alkali metal elements, found in the first column of the periodic table, with any of the halogen elements, found in the seventh column of the periodic table. Neglecting the compounds of the unstable elements francium and astatine, which have short radioactive half-lives, there are exactly twenty alkali halides, all of which have been extensively studied by physicists and chemists. The alkali halides have many physical and chemical properties in common. They are all hard, transparent, insulating, high-melting-point solids that exhibit one of two cubic crystal structures. In molten form, or when dissolved in water or another solvent with polar molecules, they are extremely good conductors of electricity. In the gas phase, which is formed at very low pressure or high temperature, they exist as polar diatomic molecules.

The alkali metals are those elements for which the individual atoms possess a single valence electron, which is easily removed. The halogens are those elements in which the valence shell is complete except for a single electron. Since only a small amount of energy is required to remove the valence electron from an alkali metal atom to form a positively charged ion (cation), while a much greater amount of energy is released when a halogen atom gains an extra electron to form a negatively charged ion (anion), bringing an alkali metal and a halogen into contact results in the rapid formation of the alkali halide with the release of a substantial amount of heat.

Although the precise behavior of the electrons and nuclei in a crystal is governed by quantum mechanics, the electrons in an alkali halide are so tightly bound to their individual ions that many characteristics of these materials can be understood based on a very simple picture, in which the ions are considered to be hard, solid spheres with fixed positive or negative charge. In this picture, each type of ion has a characteristic radius, and the distance between neighboring ions in a crystal lattice is very nearly the sum of the radii of the atoms. In one widely accepted scheme, the ionic radii for lithium, sodium, potassium, rubidium, and cesium cations are 0.60, 0.95, 1.33, 1.48, and 1.69 angstroms, respectively, while the radii for fluoride, chloride, bromide, and iodide anions are 1.36, 1.81, 1.95, and 2.16 angstroms, respectively. In general, the cation radii tend to be smaller than the anion radii, since the effect of the electrical repulsion between electrons is diminished when a cation is formed but augmented when an anion is formed. With the exception of the lithium halides, the distance between neighboring ions agrees with the sum of their ionic radii to an accuracy of 2 percent. The lithium halides are discussed below.

The crystal structure, or lattice, assumed by the alkali halides, and all other ionic crystals, reflects the balance established between the electrical attraction of the oppositely charged ions and a short-range repulsion, sometimes called the exchange repulsion, between the outer-shell electrons of neighboring ions, which is a consequence of the Pauli exclusion principle of quantum mechanics. This exchange repulsion is very small when the distance between ions is larger than the sum of their ionic radii, but rapidly becomes large when the separation becomes less than this sum.

Alkali halide crystals are cubic in character, with one of two internal arrangements of ions. Cesium chloride, cesium bromide, and cesium iodide adopt a crystal structure in which each ion sits in the middle of a cubic arrangement of eight ions of the opposite charge. All the other alkali halides have the sodium chloride structure, in which each ion is surrounded by six symmetrically placed ions of the opposite kind. Under high pressure, the chloride, bromide, and iodide of rubidium change from the sodium chloride structure to the cesium chloride structure.

Because of the small size of the lithium ion, the crystal structure of the chloride, bromide, and iodide of lithium involves contact, and therefore exchange repulsion, between the negatively charged ions, and leads to a spacing between ions about 10 percent greater than would be calculated from the ionic radii alone.

The alkali halides all have relatively high melting points, the lowest being that of lithium iodide, about 450 degrees Celsius, and the highest, just under 1,000 degrees Celsius, that of sodium flouride. The alkali halides expand on melting to form liquids that are good conductors of electricity as a result of the mobility of the alkali and halide ions. Passage of an electrical current through these liquids, sometimes called fused salts, results in the production of the alkali metal and halogen in elemental form. Boiling points of the alkali halides range from about 1,175 degrees Celsius, for lithium iodide, to just under 1,700 degrees Celsius, for sodium fluoride.

In the gas phase, the alkali halides exist as diatomic molecules, bound together by the sharing of electrons rather than the complete transfer of an electron from the metal atom to the halogen atom. The bond distances found in the alkali halide gas molecules are somewhat shorter than the corresponding interionic distances in the crystal. In sodium chloride, for example, the diatomic molecule has an interatomic distance of 2.361 angstroms, while the crystal has an interionic distance of 2.814 angstroms. The single chemical bond in the alkali halide molecule is best described as a covalent (shared electron pair) bond with substantial ionic character, ranging from about 43 percent for lithium iodide to 94 percent for cesium fluoride.

In the crystalline state, alkali halides are at most very weak conductors of electricity.

The alkali halides are strong electrolytes, however, and when dissolved in water dissociate essentially completely to form aqueous solutions that are good conductors of electricity. The solution process is aided by the large dipole moment, or separation of the centers of positive and negative charge, of the water molecule. This allows the anions to be surrounded by the positive ends of the water molecules, and the cations to be surrounded by the negative ends of the water molecules. The smaller ions, the cations of lithium, sodium, and potassium and the fluoride anion, are in fact known to bind the solvent molecules rather strongly and travel with them through the solution when an electric field is applied.

The crystalline alkali halides are transparent to visible light because the energy required to excite electrons from their normal states to the lowest available unoccupied state is greater than that carried by the photons of visible light. In the language of the band theory of solids, the energy gap between the valence band, corresponding roughly to the outermost filled level of the chloride ion, and the conduction band, corresponding roughly to the lowest unfilled level of the sodium ion, is of the order of several electronvolts. Ultraviolet radiation is sufficiently energetic to excite electrons to the conduction band of an alkali halide crystal and will enable the crystal to conduct electricity through the motion of the excited electrons, a phenomenon known as photoconductivity. Alkali halides with the sodium chloride structure absorb infrared light over a characteristic wavelength region, with the absorbed energy increasing the amount of vibration of the ions in the lattice. The optical properties of the alkali halides can be altered substantially through the introduction of appropriate impurities or other defects into the crystal structure.

Applications

The alkali halides are among the simplest ionic compounds known and have thus received a large amount of attention from chemists and physicists as models for the behavior of other ionic compounds. Some of the most interesting properties of the alkali halides concern the defects that may exist in the crystal structure. Alkali halides are prone to a type of disorder in which individual anion and cation sites in the crystal structure are left vacant. This particular type of isolated, or point, defect in the crystal structure is called a Schottky defect. In a crystal with no other form of disorder, the number of anion Schottky defects must equal the number of cation Schottky defects in order that the electrical neutrality of the crystal not be disturbed. The number of Schottky defects increases with temperature, being quite small at room temperature and approaching about 1 percent of the anion and cation sites at the melting point.

The limited electrical conductivity possessed by the alkali halides in the crystalline state is primarily the result of Schottky defects. Schottky defects are able to move by a process in which a neighboring ion of the appropriate type moves into the vacant defect site, leaving its own site vacant. The concentration of Schottky defects can be altered by introducing ions with a charge that is different from that of the ions they displace. These ions are called aliovalent ions.

If, for example, a small amount of calcium chloride is present in a sodium chloride crystal, the calcium ions, which carry two units of positive charge, are known to occupy the sites usually occupied by the singly charged sodium ions. To maintain electrical neutrality, additional vacant sodium sites must be created, or the number of existing vacant anion sites must be reduced; that is, every calcium ion must be matched by an additional cation Schottky defect or one fewer anion Schottky defect. The equilibrium between Schottky defects is such that the product of the concentration of cation Schottky defects and the concentration of anion Schottky defects is constant at any given temperature. Thus, a fivefold increase in cation Schottky defects, while it will reduce the number of anion Schottky defects to one-fifth the original value, will still result in an overall increase in the number of mobile defects and the electrical conductivity.

A second type of point defect, which occurs to a lesser extent than the Schottky defect in alkali halides, is the Frenkel defect, in which one of the smaller ions in the crystal is removed from its normal site and placed at an interstitial position, a site at which no ion normally occurs, in the crystal structure. Frenkel defects are the dominant form of disorder in other ionic crystals, notably the silver halides.

When a sodium chloride crystal is exposed to sodium metal vapor, a deep yellow coloration is observed to form at the exposed surface and to diffuse slowly into the crystal. If high voltage and a pointed electrode are used to inject electrons into the crystal, a similar colored region is observed to form at the electrode, along with the release of chlorine gas at the electrode.

These processes serve to alter the stoichiometry, or numerical balance, of sodium and chloride atoms, in the material. In the present case, additional sodium ions are inserted or some chloride ions removed, with the additional positive charge being counterbalanced by the addition of electrons, which occupy vacant chloride ion sites. These electrons trapped at vacant sites are known as color centers, or F centers (from the German Farbenzentren), and are responsible for the coloration that appears in the crystal. F centers can be introduced into the other alkali halides, producing a variety of different colors.

A number of other defect centers can be formed in alkali halides by exposure to radiation or to other environments. Sodium chloride crystals that are heated in chlorine gas develop a coloration as a result of the formation of V centers, in which a diatomic, singly charged ion occupies two adjacent chloride ion sites. Also known are so-called H centers, in which the same diatomic ion is squeezed into a single chloride site. Comparable defects are known in the other alkali halides.

In addition to the types of point defect mentioned above, a number of impurity-causing ions have been introduced into alkali halides to change their optical properties. Thallium, for example, can be introduced into the potassium chloride lattice as a singly charged cation, retaining two of its three valance electrons. These valence electrons permit the impurity to absorb light of a longer wavelength than the pure crystal and reemit the light at an even longer wavelength. Other impurities introduce electronic energy levels close to the conduction band and allow the alkali halides to become photoconductive when exposed to visible light.

Context

Sodium chloride, common salt, is essential for life. It occurs naturally as the cubic mineral halite, or rock salt. Herbivorous animals will seek out a salt lick to supply the sodium otherwise missing from their diet. Civilizations must devise ways to meet the need for salt of their population. The term "salary" is derived from the salt money paid to Roman troops. The sixteenth-century Swiss physician and alchemist Paracelsus and his followers attempted to replace the four traditional elements of Aristotelian philosophy, air, earth, fire, and water, with a trio of principles, sulfur, mercury, and salt. While the mystical and philosophical considerations that guided these primitive chemists in their investigations have little bearing on modern chemistry, their terminology may still be reflected in the chemist's use of the term "salt" to denote any pure compound produced by the neutralization of an acid with a base.

Sodium chloride occurs naturally in halite deposits, some of which have been mined continuously for centuries. It also may be prepared by the evaporation of seawater. Natural deposits of potassium chloride are also known. The electrolysis of molten, or fused, sodium chloride produces metallic sodium and chlorine gas for industrial uses. The electrolysis of brine, a concentrated aqueous solution of sodium chloride, is also done commercially to produce sodium hydroxide, or lye, and chlorine gas. The majority of the alkali halides are prepared by allowing a halide compound, such as barium chloride, to react with the hydroxide or carbonate of the alkali metal to produce the water-soluble alkali halide and an insoluble compound.

Contemporary scientific interest in the alkali halides arises from their being among the simplest ionic compounds. The alkali metal cations and halide anions exist in only one charge state and do not have any partially filled inner shells of electrons. Covalent bonding is relatively unimportant for these compounds, and the cubic structure is the easiest to treat from a theoretical standpoint. The ability to tailor the optical properties of the alkali halides through the controlled addition of impurities makes them important compounds in the study of the interaction of radiation and matter, and some specially doped alkali halides are used in detectors for γ radiation.

Principal terms

ALKALI METAL: the elements lithium, sodium, potassium, rubidium, cesium, and francium (but not hydrogen), which appear in the first column of the periodic table

BAND GAP: the minimum energy required to release an electron from a particular atom in a crystal and allow it to move about the crystal

CESIUM CHLORIDE STRUCTURE: the crystal structure assumed by the chloride, bromide, and iodide of cesium under normal conditions, in which each ion is surrounded by eight ions of the opposite charge

EXCHANGE REPULSION: a short-range repulsive force that is encountered when the outer electron shells of neighboring ions overlap; a consequence of the Pauli exclusion principle of quantum mechanics

HALOGEN: the elements fluorine, chlorine, bromine, iodine, and astatine, which appear in the seventh column of the periodic table

ION: an atom or molecule that has either gained one or more electrons to have a negative charge (anion) or lost one or more electrons to have a positive charge (cation)

NONSTOICHIOMETRY: a deviation of the ratio of ions present in a crystal from that indicated by the chemical formula

POINT DEFECT: an imperfection in the structure of a crystal in which an atom, molecular ion, or electron occurs at a point not consistent with the crystal structure

SCHOTTKY DEFECT: a defect in an ionic crystal in which a cation or anion site is unoccupied

SODIUM CHLORIDE STRUCTURE: the crystal structure assumed by almost all alkali halides, in which each ion is surrounded by six ions of the opposite charge

Bibliography

Bockris, John O'M., and A. K. N. Reddy. MODERN ELECTROCHEMISTRY. New York: Plenum Press, 1973. This ambitious volume covers a wide range of topics, including a rather thorough discussion of ionic compounds in solution and the behavior of fused salts. Presents electrochemistry not as a specialized field of chemistry but as an interdisciplinary field with strong connections to physics, chemistry, materials science, and engineering. It also is one of the few textbooks in the field that is written at a level accessible to individuals who have completed only introductory-level work in physics and chemistry.

Bronowski, Jacob. THE ASCENT OF MAN. Boston: Little, Brown, 1973. This volume, and the video series of the same name, by the prominent mathematician and humanist, presents the evolution of modern science in its cultural and historical context. The structure of crystals is discussed from mathematical, physical, and chemical perspectives at several points throughout the book.

Feynman, Richard P., Robert B. Leighton, and Matthew Sands. THE FEYNMAN LECTURES ON PHYSICS. 3 vols. Reading, Mass.: Addison-Wesley, 1963-1965. This comprehensive set of lectures by one of the leading theoretical physicists of the mid-twentieth century (Feynman) is an attempt to convey both modern and classical physics to beginning university students. The properties of crystalline materials are discussed in some detail in the second volume of these lectures.

Kittel, Charles. INTRODUCTION TO SOLID-STATE PHYSICS. New York: John Wiley, 1966. One of the standard introductions to the physics of solids. The chapters on crystal binding, optical phenomena in insulators, and point defects will provide additional information on the physical characteristics of alkali halides.

Moore, Walter J. SEVEN SOLID STATES. New York: W. A. Benjamin, 1967. This brief book is intended as a supplement to introductory chemistry texts, to provide additional materials on solid state chemistry, a topic sometimes neglected in these general works. The first chapter is devoted to sodium chloride as a prototype ionic crystal.

Morgan, Alfred. ADVENTURES IN ELECTROCHEMISTRY. New York: Charles Scribner's Sons, 1977. This is a remarkably complete discussion of the basic ideas in the field, including both basic principles and industrial applications, written especially for young readers. Describes the basic concepts of ionic solution and the role played by alkali halides in many industrial processes.

Mott, Neville H., and R. W. Gurney. ELECTRONIC PROCESSES IN IONIC CRYSTALS. New York: Dover, 1964. This is a reprint of one of the first books to be published on both the ionic and electronic properties of ionic crystals. Although intended for advanced students, the presentation of ideas is very clear and can be followed, for the most part, by students at the introductory level.

Pauling, Linus. GENERAL CHEMISTRY. New York: Dover, 1988. This is a reprint of a classic text last revised in 1970, by a distinguished chemist and educator. Chapter 6 provides a compact discussion of ionic solids in the context of a general discussion of chemical bonding.

Pauling, Linus. THE NATURE OF THE CHEMICAL BOND. Ithaca, N.Y.: Cornell University Press, 1960. A classic treatment of all aspects of chemical bonding by one of the founders of modern structural chemistry. Chapter 13 discusses ionic bonding and the alkali halides in general, with emphasis on the systematic variation in properties from one compound to another.

The Periodic Table and the Atomic Shell Model

Defects in Solids