Salts

Type of physical science: Chemistry

Field of study: Chemical compounds

Ionic salts are polar-bonded combinations of charged atoms that are found extensively throughout nature, in living organisms, the oceans, the earth's crust, and interstellar space. Such salts often are the products of acid-base chemical reactions.

Overview

A salt is a substance that is produced from the ionic bonding between cations and anions. A cation is a positively charged atom or molecule; an anion is a negatively charged atom or molecule. There are hundreds of different combinations of cations and anions that produce salts. These substances are found throughout the world and prove to be very useful for many human activities and technologies. They are particulate and granular in structure. They interact with other substances in a variety of ways, depending upon their inherent physical and chemical properties.

On the atomic level, the behaviors of cations and anions in forming salts are readily evident from the distribution of cationic and anionic atoms across the periodic table of the elements. The periodic table, first developed by the Russian chemist Dmitri Ivanovich Mendeleev in 1870, is an arrangement of the known naturally occurring and artificially produced elements. The elements are divided into eighteen vertical columns, called groups, and seven horizontal rows, called periods. All elements within a group have extremely similar physical and chemical properties (for example, melting point, boiling point, density, and chemical reactivity). The horizontal periods represent increasing energy level arrangements of electrons per atom of higher elements. The entire periodic table also is arranged from left to right and from top to bottom in order of increasing elemental atomic numbers, or increasing size and number of electrons per atom.

In terms of cations and anions, certain vertical groups on the periodic table fall within distinct classes. Atoms of elements within groups IA and IIA are distinctively cationic when they ionize. The atoms of the transition metals (periodic groups IB through VIIIB) also are distinctively cationic, although there is variance in the level of positive charge for atoms of each transition metal element. Atoms of elements within groups VIA and VIIA are distinctively anionic; they are also nonmetals. Atoms of groups IIIA through VA have a mixture of cationic and anionic properties. The group VIIIA elements are the inert, or noble, gases; these gaseous atoms do not ionize, and they rarely react with other substances, because of the great stability of their outermost electronic atomic energy levels.

Cationic elements are located to the left side of the periodic table. These elements consist of atoms having excess electrons located in unstable outer energy levels. Within any atom, three subatomic particles exist: positively charged protons, zero-charge neutrons, and negatively charged electrons. All protons and neutrons are massed in the central nucleus of an atom. Electrons orbit around the atomic nucleus within distinct energy levels, in accordance with the quantum mechanical theory of the atom. A maximum of two electrons can occupy an innermost first energy level, the energy level closest to the atomic nucleus. A maximum of eight electrons can occupy the second energy level, a maximum of eighteen electrons can occupy the third energy level, and a maximum of thirty-two electrons can occupy the fourth, fifth, sixth, and seventh energy levels.

An atom of an element is electrically neutral because it contains the same number of positively charged protons as negatively charged electrons. For example, sodium (Na) has an atomic number of eleven; it contains eleven protons and eleven electrons per atom. Within a sodium atom, the eleven electrons are arranged with two electrons in the first energy level, eight electrons in the second energy level, and the remaining electron forming the third energy level. A maximum of eighteen electrons can exist in the third energy level, however. As a result, a sodium atom will be slightly unstable. To relieve this instability, sodium atoms will tend to lose this outermost, lone third-energy-level electron whenever a chemical reaction occurs. Sodium atoms will thus undergo oxidation. When the outer electron has been lost from a sodium atom, a negative charge is lost; therefore, sodium will have a +1 charge when it ionizes in a chemical reaction. All the elements in group IA, (for example, hydrogen, lithium, sodium, potassium, rubidium, cesium, and francium) have a +1 charge when they ionize for the same reason: Atoms of each of these elements have only one electron located in their outermost electronic energy levels. They are unstable; they lose the extra electron, and positively ionize (that is, oxidize) to become cations.

In a similar fashion, periodic group IIA elements (for example, beryllium, magnesium, calcium, strontium, barium, and radium) are cationic and have a +2 charge. Atoms of these elements are slightly unstable because their outermost energy levels contain only two electrons. For these atoms, two negative charges are lost during oxidation, thereby producing two positive charges for the atoms when they ionize in chemical reactions.

The transition metal elements (periodic groups IB through VIIIB) include iron, cobalt, tungsten, nickel, copper, zinc, gold, platinum, and silver. These atoms ionize, but to varying degrees. For example, iron cations can have a +2 or +3 charge and are called iron (II) and iron (III), respectively. The other transition metals behave similarly. This variable cationic charge enables the transition metals to bond ionically with anions in several possible combinations. It also enables these metals to react with other substances (for example, ammonia and water) to produce complex cations, which may or may not have color as a result of the scattering of light through the rearranged energy levels of these atoms.

The periodic group VIIA elements (for example, fluorine, chlorine, bromine, iodine, astatine, and sometimes hydrogen) are anionic, with a -1 charge. Atoms of these elements are only one electron short of filling their outermost energy levels. Consequently, atoms of these elements, collectively known as the halogens, are extremely reactive with other substances in order to obtain the needed electron. For example, chlorine (Cl) has an atomic number of 17; an atom of chlorine has two electrons in its innermost, first energy level, eight electrons in its second energy level, and seven electrons in its third energy level. Now, the third energy level requires a maximum of eighteen electrons, which are arranged hierarchically in the s, p, and d orbitals of the third energy level. A chlorine atom really only needs to fill its last, third energy level p orbital in order to obtain stability. Consequently, a chlorine atom will react with other atoms in order to gain this one needed electron, thereby gaining a negative charge and becoming an anionic -1. This gain of electrons by an atom is termed reduction.

Atoms of periodic group VIA elements are anionic for the same reason, except their charge is -2: Each of these atoms is two electrons short of filling its outermost energy level. These elements include oxygen, sulfur, selenium, tellurium, and polonium. Like the group VIIA halogens, the group VIA elements are extremely reactive. Both the group VIA and the group VIIA elements seek to gain electrons in order to become stable like the group VIIIA inert gases (for example, helium, neon, argon, and krypton). Therefore, the very reactive group VIA and VIIA elements have high electronegativities: They tend to steal electrons from other atoms.

A prominent source of electrons for the anionic group VIA and VIIA elements is the cationic elements, which need to lose electrons, periodic groups IA and IIA in particular. When atoms of groups IA or IIA react with atoms of groups VIA or VIIA, there is a smooth transfer of electrons with accompanying ionization of the donor and recipient atoms. The cationic donors will be oxidized; the anionic recipients will be reduced. For example, a group IA sodium (Na) atom will lose an electron to a chlorine (Cl) atom from group VIIA, thereby producing the cationic Na⁺ and the anionic Cl^-. The opposite polarities, or charges, of these ions will electrically hold them together in a type of chemical bond called an ionic bond. The result of an ionic bond between two substances, usually between a cationic metal and a group VIA or VIIA anion, is an ionic salt. In this example, the ionic salt is table salt: sodium chloride (Na⁺Cl^-).

Other types of salts will include many possible combinations between group IA and group VIIA elements. In addition to sodium chloride, there are potassium chloride (K⁺Cl^-), lithium chloride (Li⁺Cl^-), cesium chloride (Cs⁺Cl^-), sodium fluoride (Na⁺F^-), potassium fluoride (K⁺F^-), and sodium iodide (Na⁺I^-), among others. Interactions between group IIA and VIA elements will produce molecules such as magnesium oxide (Mg²⁺O^2-), barium sulfide (Ba²⁺S^2-), and calcium sulfide (Ca²⁺S^2-).

The ratio of cations to anions is 1:1 in the above examples. In ionic bonds between group IIA and group VIIA elements, the ratio of cations to anions is 1:2. Examples include magnesium chloride (MgCl₂), calcium chloride (CaCl₂), barium chloride (BaCl₂), magnesium fluoride (MgF₂), and barium iodide (BaI₂). In ionic bonds between group IA and group VIA elements, the ratio of cations to anions is 2:1. Examples include sodium sulfide (Na₂S), potassium sulfide (K₂S), cesium sulfide (Cs₂S), and lithium oxide (Li₂O).

The transition metals produce complex ionic molecules having more complicated cation-to-anion ratios. An example is iron (II) chloride (FeCl₂), in which the +2 charge of the one iron cation must be counterbalanced by two -1 charged chlorine anions. Another example is iron (III) chloride (FeCl₃), in which a +3 iron cation is balanced by three -1 chlorine anions. Furthermore, ionic bonding can occur between cationic and anionic atoms and molecules. Examples include ammonium chloride (NH4⁺Cl^-) and sodium hydroxide (Na⁺OH^-). Molecular cations and anions have lost or gained electrons, respectively, during the formation of covalent bonds between their constituent atoms.

Regardless of their molecular form, ionic salts are the common products of many diverse chemical reactions that occur in nature. Among the principal chemical reactions on earth are corrosion, acid-base interactions, and oxidation. These reactions bring together metals with nonmetals to produce cationic-anionic salts. Salts form from oceans to mountaintops, and they are utilized by humans for a variety of purposes.

Applications

Ionic salts are the principal reaction products of acid-base interactions. As a result, they play an integral part in acid-base chemical reactions used by industry, in the operation of batteries, and in natural physical processes. The survival of living organisms also hinges on the presence of adequate amounts of certain ionic salts in the body tissues. Modern medical treatments for a variety of human diseases and disorders involve the regulation of bodily levels of cations, anions, acids, and bases.

An acid is a substance that donates hydrogen cations to other substances during chemical reactions. A hydrogen cation is nothing more than a single proton. Acids are characterized by their hydrogen content, and by the occurrence of ionic bonding between periodic groups IA or IIA and VIA or VIIA. Some examples of acids include hydrochloric acid (H⁺Cl^-), sulfuric acid (H₂SO₄ or H⁺H⁺SO₄^2-), phosphoric acid (H₃PO₄ or H⁺H⁺H⁺PO₄^3-), acetic acid (H⁺C₂H₃O₂^-), and carbonic acid (H₂CO₃). In each case, a hydrogen or several hydrogens are ionically bonded to an anion.

A base is a substance that accepts hydrogen cations from other substances during chemical reactions. Bases, like acids, are characterized by the presence of ionic bonding between periodic group IA or IIA cations and group VIA or VIIA anions. Bases usually contain very little hydrogen, however. Some examples of bases include sodium hydroxide (Na⁺OH^-), cyanide (CN-), sulfate ion (SO₄^2-), and phosphate ion (PO₄^3-).

A base is a substance that accepts hydrogen cations from other substances during chemical reactions. Bases, like acids, are characterized by the presence of ionic bonding between periodic group IA or IIA cations and group VIA or VIIA anions. Bases usually contain very little hydrogen, however. Some examples of bases include sodium hydroxide (Na⁺OH^-), cyanide (CN^-), sulfate ion (SO₄^2-), and phosphate ion (PO₄^3-).

Both acids and bases can be very corrosive, illustrating the extreme reactivity of some ionic molecules. For example, the base sodium hydroxide etches glass. When acids and bases chemically react with one another, the reaction products usually include an ionic salt, such as NaCl, KCl, or NaF. When an acid gives away a proton, the acid itself becomes a base. Likewise, when a base accepts a proton, the base becomes an acid. Therefore, acids and bases are conjugates of one another when they rearrange their ionic bonds.

The degree of acidity or basicity and, therefore, the degree of corrosiveness of an acid or base is measured using the pH scale, a measure of the hydrogen cation concentration in a particular substance. The pH scale ranges from 0 to 14, with 7 being neutral, the pH of pure water (H₂O). A substance whose pH falls between 0 and 7 is an acid; the lower the pH of the substance, the greater is its acidity. A substance whose pH falls between 7 and 14 is a base; the higher the pH of the substance, the greater is its basicity.

Within the human body, ionic salts, acids, and bases are maintained at specific, balanced levels for proper physiological functions. Disruption to this delicate balance can cause illness, permanent cell death and tissue damage, and very possibly death to the entire organism. Within the stomach, hydrochloric acid wears away food particles during digestion; a protective layer of mucus lines the inner stomach wall to protect the stomach tissue from its own acid. Within the bloodstream, the concentrations of cations, anions, ionic salts, acids, and bases are carefully regulated by organs such as the liver and kidneys so that the blood pH remains between 7.35 and 7.45. If the blood pH strays from this range, the affected person could go into shock and die. Hospital patients who are recuperating from surgery or illness are routinely administered intravenous fluids such as saline in order to maintain a proper balance of electrolytes (ions) in the bloodstream.

In batteries and in industrial processes such as electrolysis and electroplating, ionic salts, acids, and bases are critical ingredients. Their chemical reactivity in solution results in ionization, the transfer of electrons, and the formation of positive and negative electrodes, or poles, on the battery. The positive (cathode) and negative (anode) cells of the battery are linked by an ionic salt bridge to facilitate the chemical ionization reactions and the transfer of electrons. The electron flow will drive the motor of a machine that is connected to the poles of the battery.

In the environment, the erosion of soil results in the deposition of ionic salts in rivers, oceans, and coastal areas. Furthermore, the industrial removal of important substances (for example, iron, copper, and gold) from the earth usually requires the chemical removal of the pure metal from its impure ionic ore. Also, rainwater is slightly acidic; the pollutants that we release into the atmosphere from our cars and industries increase the acidity of rain, thereby damaging plant and animal life wherever that rainwater falls. Forested areas in the eastern United States and southeastern Canada have been devastated by the effects of acid rain caused by American industrial pollution.

Context

Ionic salts are chemically bonded cations and anions. Cations are positively charged atoms or molecules that have lost an electron or electrons from their unstable outermost atomic energy levels. On the periodic table of the elements, cations include atoms of metallic elements in groups IA and IIA, and the transition metals in groups IB through VIIIB. Anions are negatively charged atoms or molecules that tend to gain an electron or electrons in order to fill and stabilize an unstable outer atomic energy level. Anions are located among the periodic group VIA and VIIA nonmetals, located to the right side of the periodic table.

When a metal and a group VIA or VIIA element interact in chemical reactions, each element ionizes. The metal loses electrons to become a cation; the group VIA or VIIA nonmetal gains these electrons to become an anion. The oppositely charged atoms will attract each other by an electrical bond called an ionic bond. Such a bond is weaker than the very strong, electron-sharing covalent bonds that occur between many elements during molecule formation.

Ionization energy, the tendency to ionize, and electronegativity, the ability to attract electrons, are two phenomena that increase to the right and to the top of the periodic table of the elements. These physical properties are responsible for the high chemical reactivity of the group VIIA halogens, in particular fluorine and chlorine. As a result, these substances are very effective as antimicrobial agents and are used in treating drinking water, swimming pool water, and wastewater. As ionic salts, they are very effective in seasoning and preserving food. The group VIA elements are highly reactive for the same reasons.

The formation of cations and anions from unstable elements and their subsequent interaction to form ionic bonds are a natural progression of the evolution of the universe. Ionic salts constitute a major component of the crusts of the inner rocky planets of our solar system. These planets are characterized by their high iron-silicate makeup. On earth, the erosion of solid landforms leads to the deposition of ionic salts in the oceans. Excess salt precipitates out of seawater to help form beaches, dunes, and salt brines. When oceans or large lakes dry up, deserts form, consisting of sand and salt flats.

Life presumably originated and evolved in the salty oceans of earth. The ionic salt content of the cells of marine organisms is virtually identical to their saltwater surroundings. Even the land animals that evolved from marine ancestors have cellular salt concentrations very similar to that of the oceans. The ionic salt concentrations within all organisms are carefully maintained so that a balanced cellular pH is managed. Virtually all living organisms have physiological pH ranges between 7.1 and 7.8, slightly basic, like the oceans. There are some microorganisms, such as the halophilic bacteria, that thrive in high ionic-salt environments.

Principal terms:

ACID: a substance that loses a proton (hydrogen cation) during a chemical reaction, usually donating the proton to a chemical base

ANION: a negatively charged atom, compound, or molecule that has gained an electron to stabilize an outer atomic energy level

ATOMIC RADIUS: the radius of an atom; the distance from the center of an atomic nucleus to the outermost orbiting electrons of the atom

BASE: a substance, also called an alkali, that gains a proton (hydrogen cation) during a chemical reaction, usually receiving the proton from an acid

CATION: a positively charged atom, compound, or molecule that has lost an electron from an unstable outer atomic energy level, thereby eliminating that energy level

ELECTRONEGATIVITY: the tendency of certain substances to attract electrons more strongly than other substances; elements located to the upper right of the periodic table of elements have high electronegativities

ENERGY LEVEL: a particular orbit of electrons about the nucleus of an atom; each energy level of an atom contains a specific maximum number of electrons having specific quanta of energy

IONIC BOND: a polar electrical attraction between oppositely charged substances (cations and anions) resulting from the transfer of electrons between these substances

OXIDATION: a process in which a substance loses electrons from its outermost atomic energy levels

REDUCTION: a process in which a substance gains electrons from other molecules

Bibliography

Bell, R. P. THE PROTON IN CHEMISTRY. Ithaca, N.Y.: Cornell University Press, 1959. This short book is a thorough survey of ionic molecular structure and acid-base chemical reactions. Excellent diagrams, graphs, and reference lists support a clearly written text. A knowledge of general chemistry and algebra would be useful for following chemical reactions, reaction rates, and pH calculations.

Butler, J. N. SOLUBILITY AND PH CALCULATIONS. Reading, Mass.: Addison-Wesley, 1964. This brief work is a thorough but very understandable presentation of acid-base chemistry for the beginning chemistry student. The book provides excellent examples and illustrations, plus problem sets for practice. The work is an indispensable manual for chemists at any level of training.

Companion, Audrey L. CHEMICAL BONDING. New York: McGraw-Hill, 1964. This excellent book is a detailed but very brief, well-written introduction to the formation of chemical bonds between atoms. Chapter 5, "Ionic, Metallic, and Van der Waals Bonding," discusses the formation of polar ionic bonds between cations and anions. Very clear tables and diagrams are provided for the beginning chemistry student.

Coulson, C. A. VALENCE. 2d ed. London: Oxford University Press, 1961. This comprehensive work is a detailed study of the arrangement of electrons around atoms and how these electrons interact during various types of chemical bonding between atoms. Ionic bonding is described in detail. While the book is intended for the advanced chemistry student, the early chapters on valence bond theory are very clear for the lay person.

Gillespie, Ronald J., David A. Humphreys, N. Colin Baird, and Edward A. Robinson. CHEMISTRY. 2d ed. Boston: Allyn & Bacon, 1989. This lengthy introductory chemistry book for undergraduate chemistry majors is clearly written and beautifully illustrated. All major subject areas within chemistry are described by many good examples, problems, and illustrations. Chapter 14, "Acid-Base Equilibria," is a detailed discussion of acid-base chemistry and the properties of ionic salts.

Hamer, Walter J. THE STRUCTURE OF ELECTROLYTE SOLUTIONS. New York: John Wiley & Sons, 1959. This book is a very thorough discussion of cations, anions, ionic salts, acids, bases, and other electrolytes and their properties in solution. Important experiments are described along with graphical displays of data and extensive reference lists. Some chapters require a strong mathematical background, although the text throughout is very understandable.

MacIntyre, Ferren. "Why the Sea Is Salt." In THE PHYSICS OF EVERYDAY PHENOMENA. San Francisco: W. H. Freeman, 1979. This review article for a general audience, first published in 1970, is an excellent discussion of chemical and physical processes on earth that create ionic salts and deposit these salts into the sea. Ionic salts are described with great detail and clarity. Several valuable data tables are also provided.

Pauling, Linus. THE NATURE OF THE CHEMICAL BOND. Ithaca, N.Y.: Cornell University Press, 1960. This classic book, written by a double Nobel laureate, is the comprehensive summary of the chemical properties of various substances. Special emphasis is placed upon the structural bonding of molecules and compounds, including covalent, ionic, and hydrogen bonds.

Acids and Bases