Alkali Metals | Research & Encyclopedia Articles

Alkali Metals

The alkali metals are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). All of the alkali metals are found in group IA (1) of the periodic table of the elements.

Because chemical properties are determined by the outer electron configuration of elements, it is important to note that the valence (outer) shell of all alkali metals contains a lone s¹ electron. Lithium has a 2s¹ outermost electron. Sodium has a 3s¹ outermost electron. Potassium has a 4s¹ outermost electron. Rubidium has a 5s¹ outermost electron. Cesium has a 6s¹ outermost electron. Francium has a 7s¹ outermost electron.

All of the alkali metals seek to lose their outermost s¹ electron in to achieve the electron configuration of the nearest noble gas. Because they lose their valence electron easily, these elements are very reactive. In nature, alkali metals are found only as ions (charged particles formed when atoms gain or lose electrons) in compounds, never as free metals.

The atomic properties of the alkali metals vary according to well-established periodic trends regarding chemical and physical properties.

With regard to group IA (1) on the periodic table, the alkali metals increase in atomic radius as one moves down the group. As a result, lithium has the smallest atomic radius and francium has the largest.

Because the first ionization energy (energy required remove an element's outermost electron) decreases down a group on the periodic table, lithium has the highest first ionization energy (ionization potential) and francium the lowest. It requires far less energy to remove the 7s¹ electron from francium than to remove the 2s¹ electron from lithium.

All alkali metals have relatively low melting points. The melting point of alkali metals lowers as one moves down group IA (1) on the periodic table. The melting point of lithium is 393.062°F (200.59°C). Francium's melting point is only 80.69°F (27.05°C). Accordingly, the boiling points of alkali metals also decrease down the group.

The boiling point of lithium is 2456.33°F (1346.85°C). Cesium's boiling point is 1300.73°F (704.85°C). The boiling point for francium is undetermined.

The word alkali comes from the Arabic word for "ashes," where the oxides of these metals were first found. Alkali is now used as another word for base (see acids and bases); when the ashes containing the oxides of the alkali metals are dissolved in water, a base is formed. For example, sodium oxide dissolved in water yields sodium hydroxide, or lye.

The free element sodium was first produced in 1807 by the English chemist Humphry Davy by electrolysis of molten sodium hydroxide. He isolated potassium, another well known alkali metal, from potassium hydroxide in the same year. Lithium was discovered in 1817. Rubidium and cesium were discovered in 1860 by Robert Bunsen using the newly developed spectroscope which shows light-emission patterns that are characteristic for each element. Francium was discovered in 1939 as by the disintegration of an isotope of actinium.

The alkali metals are silver colored except for cesium, which is pale gold.

Alkali metals are so soft that they can be cut with a knife. Because the freshly exposed surface quickly oxidizes in air, these metals are stored in mineral oil or other non-aqueous solvents.

When an alkali metal is dropped into water, hydrogen is released, sometimes explosively. All of these metals dissolve in ammonia to form blue solutions that conduct electricity and function as strong reducing agents.

All alkali metals exhibit the photoelectric effect and are used in photoelectric cells. Cesium and rubidium lose their valence electrons especially easily when light strikes their polished surfaces and are photosensitive over the full visible spectrum.

Sodium compounds produce a distinctive yellow flame when burned. Potassium compounds burn with a lilac flame often masked by the stronger sodium yellow. A dark blue glass can be used to mask the yellow of sodium, allowing the potassium flame to be seen as red. Lithium compounds produce a crimson red flame. Rubidium and cesium flames are reddish violet or magenta. The compounds are used in fireworks, along with potassium perchlorate, chlorate, and/or nitrate as oxidizing agents.

Nearly all compounds of the alkali metals are soluble in water.Sodium chloride, containing ions of sodium and ions of chlorine, is well known as table salt. The global supply of sodium chloride is mostly mined as rock salt from man-made caves resulting from evaporation of ancient inland seas. Large deposits, consisting of up to 70% sodium chloride exist in a number of countries. The salt mine in Wielitzka, Poland, has been worked continuously for over 600 years. The deposit is 500 miles long, 20 miles wide, and 1200 feet thick, and includes a chapel carved into the salt. Other alkali salts are also mined. The Stassfurt mine in Germany yields potassium chloride and potassium sulfate as well as sodium chloride. Searles Lake in California, a brine-encrusted lake in the Mojave Desert, is a major source of lithium salts as well as sodium and potassium salts. Vast deposits of sodium nitrate are found in South America

Sodium and potassium compounds are leached from their parent rocks by weathering. Sodium concentrates in seas, but potassium is strongly absorbed by clays. Potassium compounds are usually derived from plants, although a deposit in Saskatchewan, Canada, formed from the crystallization of minerals in an ancient sea, is thought to contain ten billion tons of potassium chloride.

There are many industrial uses for alkali metals and their compounds.

Sodium is transported in railroad cars with heating units so that the metal can be easily loaded or unloaded as a liquid. Liquid sodium's low melting point and good heat transfer properties lead to use as a coolant in nuclear breeder reactors, especially on submarines. In sodium vapor lights, sodium vaporizes when electricity is passed through it. For most of the twentieth century, metallic sodium was heated with lead to form an alloy as the first step in the production of tetraethyl lead, an antiknock gasoline additive. Because of environmental concerns, lead in gasoline was gradually phased out in the United States and was no longer commercially available as of 1990. Sodium is also used as a chemical reducing agent in producing titanium, zirconium, niobium, and tantalum from their fused salts.

Sodium hydroxide, the most common base in teaching laboratories, is an important industrial chemical used making soap, dyes, and other chemical compounds such as sodium cyanide. Sodium hydroxide, also known as caustic soda, is used in the production of aluminum and by the pulp and paper industry. It is also a popular drain cleaner. Sodium hydroxide is produced from the electrolysis of brine, yielding chlorine as an important byproduct.

Sodium carbonate has been obtained from natural deposits since prehistoric times. Most of the sodium carbonate used in North America comes from large deposits of the mineral trona, a sodium carbonate/bicarbonate found in Wyoming. It can also be produced from sodium chloride. It is known as washing soda or soda ash, and is used for softening water, in the manufacture of borax, and in making glass, paper, detergents, and soap. Borax, or sodium borate, is used as a washing agent and as an insecticide; although effective against cockroaches and ants it is not highly toxic to mammals.

Because of the solubility of alkali metal salts, they find many uses in which it is the anion rather than the cation that is important.Sodium bicarbonate (baking soda) is used to treat excess stomach acidity. Sodiumfluoride, sodium fluorosilicate, sodium arsenate, sodium borate (borax), and sodium chlorate are all used as weed or pest-killers. Sodium chlorite, hypochlorite, perborate, and peroxide are used to bleach paper, cotton, and rayon. Sodium hyposulfite is used in the reduction of certain dyes. Sodium thiosulfate is used to dissolve unreduced silver salts in photographic processes. Sodium sulfide is used as a depilatory and in the manufacture of sulfur dyes. Sodium silicate is used to impregnate wood, to weight silk, as a mordant, and as an adhesive.Sodium benzoate is used as a food preservative. Sodium and potassium dichromate and potassium permanganate are powerful laboratory oxidizing agents. Potassium iodide is added to table salt because the iodide ion prevents goiter. Trisodium phosphate, or TSP, is a strong cleansing and disinfecting agent. Potassium chlorate is a powerful oxidizing agent and a source of oxygen in the laboratory.

Cream of tarter, potassium tartrate, is used in cooking. It is a hard, crystalline substance which forms as grapes are fermented into wine. Potassium perchlorate is the oxidizing agent used to set off fireworks.

Searles Lake supplies about one-half the world's supply of lithium, primarily as lithium chloride. Lithium has the lowest density of any metal. Lithium/magnesium alloys have an exceptionally high strength-to-weight ratio and so are important in aircraft and spacecraft design. Metallic Lithium is used in lightweight batteries.

Lithium aluminum hydride is an important reducing agent in organic chemistry. One of the largest uses of metallic lithium, industrially and in the laboratory, is in the preparation of organolithium compounds such as methyl lithium as starting compounds for organic syntheses.

Alkali metals and their associated compounds are also important to biological systems.

Sodium and potassium ions are essential components of the human nervous system and brain and are present in all body fluids and essential for proper electrolyte balance. Potassium and magnesium ions are important cellular cations. Potassium chloride is used as a salt substitute by people who, because of high blood pressure, are on a restricted sodium diet.

Sodium and potassium phosphates and nitrates are used as fertilizers. Not only is the phosphate ion necessary, but potassium is necessary in maintaining water balance. Without potassium, plants are unable to absorb water and will not grow.

Lithium carbonate is used to treat manic depression, although the mechanisms for its effectiveness is not well understood it is known that the body cannot always distinguish between lithium and sodium. In order to maintain a constant lithium level, the body must maintain a relatively constant sodium level. If sodium levels fall, more lithium is retained, sometimes even reaching toxic levels.

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