Alkaline earth metals

On the Periodic table , Group 2 (IIA) consists of beryllium, magnesium, calcium, strontium, barium, and radium. This family of elements is known as the alkaline earth metals , or just the alkaline earths. Although early chemists gave the name "earths" to a group of naturally occurring substances that were unaffected by heat and insoluble in water , the alkaline earth metals are also usually found in the continental crust . In contrast, Group 1 compounds and ions tend to concentrate in the ocean.

Calcium carbonate is geologically evident as limestone , marble , coral, pearls, and chalk—all derived mainly from the shells of small marine animals. The weathering of calcium silicate rocks over millions of years converted the insoluble calcium silicate into soluble calcium salts, which were carried to the oceans . The dissolved calcium was used by marine organisms to form their shells. When the organisms died, the shells were deposited on the ocean floor where they were eventually compressed into sedimentary rock . Collisions of tectonic plates eventually allow this rock to rise above the ocean floor to become "land-based" limestone deposits.

Caverns throughout the world are formed by the action of atmospheric carbonic acid (water plus carbon dioxide ) on limestone to form the more soluble calcium bicarbonate. When the solution of calcium bicarbonate reaches the open cavern and the water evaporates, carbon dioxide is released and calcium carbonate remains. The calcium carbonate is deposited as stalagmites if the drops hit the ground before evaporating, or as stalactites if the water evaporates while the drop hangs from above.

Other minerals of alkaline earth metals are beryllium aluminum silicate (beryl), calcium magnesium silicate (asbestos), potassium magnesium chloride (carnallite), calcium magnesium carbonate (dolomite ), magnesium sulfate (epsomite), magnesium carbonate (magnesite), hydrogen magnesium silicate (talc), calcium fluoride (fluorspar), calcium fluorophosphate (fluorapatite), calcium sulfate (gypsum ), strontium sulfate (celestite), strontium carbonate (strontianite), barium sulfate (barite), and barium carbonate (witherite). Radium compounds occur in pitchblende, which is primarily uranium oxide, because radium is a product of the radioactive disintegration of U-238. Most pitchblende in the United States is found in Colorado.

The alkaline earth metals, like the alkali metals, are too reactive to be found in nature except as their compounds; the two valence electrons completing an s-subshell are readily lost, and ions with +2 charges are formed. The alkaline earth metals all have a silver luster when their surfaces are freshly cut, but, except for beryllium, they tarnish rapidly. Like most metals, they are good conductors of electricity .

Only magnesium and calcium are abundant in Earth's crust. Magnesium is found in seawater and as the mineral carnallite, a combination of potassium chloride and magnesium chloride. Calcium carbonate exists as whole mountain ranges of chalk, limestone, and marble. Its most abundant mineral is feldspar , which accounts for two-thirds of the earth's crust. Beryllium is found as the mineral beryl, a beryllium aluminum silicate. With a chromium-ion impurity, beryl is known as emerald. If iron ions are present, the gemstone is blue-green and known as aquamarine.

Beryllium is lightweight and as strong as steel. It is hard enough to scratch glass . Beryllium is used for windows in xray apparatus and in other nuclear applications, allowing the rays to pass through with minimum absorption.

Because beryllium is rather brittle, it is often combined with other metals in alloys. Beryllium-copper alloys have unusually high tensile strength and resilience, which makes them ideal for use in springs and in the delicate parts of many instruments. The alloy does not spark, and so finds use in tools employed in fire-hazard areas. Because beryllium-nickel alloys resist corrosion by salt water, they are used in marine engine parts.

Magnesium, alone or in alloys, replaces aluminum in many construction applications because the supply of this metal from seawater is virtually unlimited. Magnesium is soft and can be machined, cast, and rolled. Magnesium-aluminum alloys (trade name Dowmetal) are often used in airplane construction.

Magnesium hydroxide is used as milk of magnesia for upset stomachs. Epsom salts are magnesium sulfate. Soapstone, a form of talc, is used for laboratory table tops and laundry tubs. Magnesium oxide is used for lining furnaces.

Slaked lime, or calcium hydroxide, is the principal ingredient in plaster and mortar, in which the calcium hydroxide is gradually converted to calcium carbonate by reaction with the carbon dioxide in the air. Slaked lime is an important flux in the reduction of iron in blast furnaces. It is also used as a mild germ-killing agent in buildings that house poultry and farm animals, in the manufacture of cement and sodium carbonate, for neutralizing acid soil , and in the manufacture of glass.

Calcium carbide, made by reacting calcium oxide with carbon in the form of coke, is the starting material for the production of acetylene. Calcium propionate is added to foods to inhibit mold growth. Calcium carbonate and calcium pyrophosphate are ingredients in toothpaste.

Plaster of Paris is 2CaSO₄•H₂O, which forms CaSO₄•2H₂O (gypsum), as it sets. Gypsum is used to make wallboard, or sheet rock. Asbestos—no longer used as a building material in the United States because of concerns that exposure to asbestos fibers can cause cancer—is a naturally occurring mineral, a calcium magnesium silicate. Calcium and magnesium chlorides, byproducts of sodium chloride purification, are used in the de-icing of roads. Calcium chloride absorbs water from the air, so is used in the prevention of dust on roads, coal , and tennis courts and as a drying agent in the laboratory.

Florapatite, a calcium fluorophosphate, is an important starting material in the production of phosphoric acid, which, in turn, is used to manufacture fertilizers and detergents. The mines in Florida account for about one-third of the world's supply of this phosphate rock. Fluorspar, or calcium fluoride, is used as a flux in the manufacture of steel. It is also used to make hydrofluoric acid, which is then used to make fluorocarbons such as Teflon.

Calcium is involved in the function of nerves and in blood coagulation. Muscle contraction is regulated by the entry or release of calcium ions by the cell. Calcium phosphate is a component of bones and teeth. Hydroxyapatite, calcium hydroxyphosphate, is the main component of tooth enamel. Cavities are formed when acids decompose this apatite coating. Adding fluoride to the diet converts the hydroxyapatite to a more acid-resistant coating, fluorapatite or calcium fluorophosphate. Magnesium is the metal ion in chlorophyll, the substance in plants that initiates the photosynthesis process in which water and carbon dioxide are converted to sugars. Calcium ions are needed in plants for cell division and cell walls. Calcium pectinate is essential in holding plant cells together. Calcium and magnesium ions are required by living systems, but the other Group 2 elements are generally toxic.

The word barium comes from the Greek barys, meaning heavy. Barium salts are opaque to x rays, and so a slurry of barium sulfate is ingested in order to outline the stomach and intestines in x-ray diagnosis of those organs. Although barium ions are poisonous, the very low solubility of barium sulfate keeps the concentration low enough to avoid damage.

Both barium and strontium oxides are used to coat the filaments of vacuum tubes, which are still used in some applications. Because these elements act to remove traces of oxygen and nitrogen, a single layer of barium or strontium atoms on a filament may increase the efficiency more than a hundred million times.

Radium is a source of radioactive rays traditionally used in cancer treatment, though other radioactive isotopes are now more commonly used. A radioactive isotope of strontium, strontium-90, is a component of nuclear fallout.

The alkaline earths and their compounds burn with distinctive colors. The green of barium, the red of strontium, and the bright white of magnesium are familiar in fireworks. Strontium is also used in arc lamps to produce a bright red light for highway flares.

See also Chemical bonds and physical properties; Chemical elements; Geochemistry; Stalactites and stalagmites

Alkaline earth metals

The elements that make up Group 2 of the periodic table are commonly called the alkaline earth metals. They include beryllium, magnesium, calcium, strontium, barium, and radium. All of these elements contain two electrons in the outermost energy level of their atoms, and they tend to have similar chemical and physical properties. Their properties can also be compared to those of the alkali metals, which lie next to them on the periodic table. They are shiny, relatively soft, and white or silvery in color.

Like the alkali metals, the alkaline earth metals react with acids and water to produce hydrogen gas. These reactions, however, are less intense than are those with the alkali metals. Alkaline earth metals also react vigorously with oxygen. Magnesium burns so actively in air, for example, that it is often used in flares because of the brilliant white light it produces during combustion.

Flame tests can be used to identify compounds of the alkaline earth metals. The characteristic colors of these elements are orangish-red for calcium, crimson for strontium, and apple-green for barium. The brilliant colors produced in fireworks displays are often produced by compounds of strontium and barium.

Beryllium

Beryllium ranks number 50 in abundance among the chemical elements. Interestingly enough, it occurs most commonly in gemstones and beautiful minerals such as beryl, emeralds, and aquamarine. The largest crystals of beryl are about a meter in length and weigh up to 60 metric tons.

The most important industrial application of beryllium is in the manufacture of alloys (metal mixtures). In very small amounts, the element adds strength, durability, and temperature stability to alloys. Copper-beryllium alloys make good hand tools in industries that use flammable solvents because the tools do not cause sparks when struck against other objects. Nickel-beryllium alloys are used for specialized electrical connections and various high temperature applications. Beryllium is used instead of glass in X-ray tubes because it lets through more of the X-radiation than glass would.

Beryllium is toxic to humans. Exposure to high concentrations can cause a pneumonia-like condition that can quickly result in death. Long-term exposure to even small concentrations can result in serious health problems, in particular a respiratory problem known as berylliosis.

Magnesium

Magnesium is the sixth most common element in Earth's crust. It occurs in minerals such as dolomite, magnesite, carnallite, asbestos, soapstone, mica, and spinel. The oceans also contain relatively high concentrations of magnesium chloride.

Magnesium performs a critical role in living things because it is a key component of chlorophyll. Chlorophyll is the green pigment that captures the energy of sunlight for storage in plant sugars during photo-synthesis. (Through the process of photosynthesis, plants use light to break down chemical compounds). Chlorophyll is a large molecule called a porphyrin; the magnesium occupies the center of the porphyrin molecule. In the animal kingdom, a similar porphyrin called heme allows hemoglobin to transport oxygen around in the bloodstream; in the case of heme, however, iron rather than magnesium occupies the central place in the porphyrin.

Elemental magnesium is a strong, light metal, particularly when alloyed, or mixed, with other metals like aluminum or zinc. These alloys have many uses in construction, such as in the manufacture of airplane

parts. Alloys of magnesium and the rare earth elements are so temperature resistant that they are used to make car engine parts.

Calcium

Calcium is the third most common metal on Earth, exceeded only by iron and aluminum, and the fifth most common element. Naturally occurring compounds of calcium include limestone, dolomite, marble, chalk, and iceland spar (all forms of calcium carbonate); gypsum (calcium sulfate); fluorite (calcium fluoride); and apatite (calcium fluorophosphate). Compounds of calcium are also found in sea water.

Calcium is an essential nutrient for living organisms. One of its function is the proper development of bones and teeth. Nutritionists say that growing children need about 1.5 grams of calcium every day to maintain good health. Calcium is also needed for the coagulation (clotting) of blood and for maintaining a normal heartbeat and blood pressure.

The industrial applications of calcium are numerous. Both limestone and gypsum have been used in building materials since ancient times; in general, gypsum was used in drier climates. Marble is also a good building material. Limestone and dolomite are the principle sources of slaked lime (calcium hydroxide) and quick lime (calcium oxide) for the steel, glass, paper, dairy, and metallurgical industries. Lime can act as an agent to remove impurities from steel, as a neutralizing agent for acidic industrial waste, as a reagent (a chemically active substance) for reclaiming sodium hydroxide from paper pulping waste, and as a scrubbing compound to remove pollutants from smokestack effluent. The paper industry uses calcium carbonate as an additive to give smoothness and opacity (the opposite of transparency) to the finished paper. The food, cosmetic, and pharmaceutical industries use it in antacids, toothpaste, chewing gum, and vitamins.

Strontium, barium, and radium

Strontium and barium are the fifteenth and fourteenth most abundant elements, respectively, in Earth's crust. They also occur in very small concentrations in the oceans. Radium is a radioactive element that occurs only in association with uranium, from which it is formed by radioactive decay. (A radioactive element is one that spontaneously gives off energy in the form of particles or waves by disintegration of their atomic nuclei.) This relationship between uranium and radium provides a reliable way to find the age of rocks. The larger the amount of radium in a rock, the longer decay has been taking place and the older the rock is.

Because of the brilliant red color they produce when burned, strontium compounds are widely used in fireworks and flares. Strontium carbonate is also a glass additive, and strontium hydroxide is a refining agent in the production of beet sugar. The most important commercial application of barium is in the form of barium sulfate, used as a lubricating mud in well-drilling operations. In the medical field, patients with gastrointestinal (stomach and intestinal) problems are often required to drink a chalky, white liquid form of barium sulfate before having X-ray examinations.

Radium was formerly used in medicine to treat various kinds of cancer and other conditions. Its use has declined, however, as safer radioactive materials have been discovered. Compounds of radium were also used to paint the luminous numbers on watch dials. That application has been stopped because of the health risks to workers who used the radium paint.

[See also Chlorophyll; Element, chemical; Periodic table ]

Alkaline Earth Metals

Alkaline earth metals are the six elements forming Group IIa in the Periodic Table: beryllium (Be), magnesium (Mg), Calcium (Ca), Barium (Ba), Strontium (Sr), and Radium (Ra)✶. Their oxides are basic (alkaline), especially when combined with water. "Earth" is a historical term applied to nonmetallic substances that are insoluble in water and stable to heating, and also the properties of the oxides. Hence, the term "alkali earths" is often used to describe these elements.

✶ See Periodic Table in the For Your Reference section of Volume 1.

Each metal has the electron configuration of an inert (noble) gas plus two electrons in the next higher s orbital. Thus, Mg is 1s ²2s ²2p ⁶3s ² or alternatively (Ne)3s ². The bonds of most compounds of alkali earths are ionic in nature because these outermost electrons are readily lost, forming stable divalent cations. Mg, however, can form compounds with both ionic and covalent bonds , whereas most compounds of Be are covalent. The heavier alkali earths are sometimes compared to Group IIb elements (zinc [Zn], cadmium [Cd], mercury [Hg]) that also have a filled s orbital (5s ²), but the filled 4d ¹⁰ orbitals and higher ionization energies of the latter make compounds of Group IIb elements markedly less ionic in character than those of alkali earths.

Mg and Ca are the eighth and sixth most abundant elements in Earth's crust at 2.5 and 3.6 percent, respectively. Be, Sr, and Ba comprise 0.001, 0.025, and 0.05 percent, respectively. Ra is radioactive, and since its longest-lived isotope ²²⁶Ra has a half-life of 1,600 years, there is very little Ra in Earth's crust. It is nonetheless present because ²²⁶Ra is continuously formed by the decay of uranium (²³⁸U). Alkali earth elements are very reactive and strongly reducing in character; thus, none occurs in a free state in the environment. They readily react with oxygen, and the pure metals tarnish in air, forming a surface layer of the oxide. The metals are soluble in liquid ammonia, forming covalent compounds with the general formula M(NH₃)₆. These solutions are strongly basic and frequently find application in industry.

Oxides of alkali earths were known in ancient times, calcium oxide being lime (from the Latin word calx ). Magnesium oxide or magnesia was also known, its name probably deriving from a district in Asia Minor. Oxides of the other alkali earths were identified in the eighteenth century. Barium oxide or baryta was found in the mineral called heavy spar and given the name barys (from the Greek, meaning "heavy"). Strontia or strontianite (strontium carbonate) was found in a lead mine at Strontian in Scotland. Beryllium oxide was extracted from the mineral beryl (from the Greek word bèryllos ). Be was originally called glucina (from the Greek glykys, meaning "sweet") because of its taste and is sometimes still referred to as glucinum in France.

The English chemist Sir Humphry Davy first isolated Mg, Ca, Sr, and Ba in 1808 by means of electrolysis. (Mg was originally called magnium since Davy had already applied the word "magnesium" to the element manganese.) Be was initially isolated from beryl by the French chemist Antoine Bussy and independently in Germany by Friedrich Wöhler in 1828. The discovery of Ra did not occur until 1898 when Marie and Pierre Curie purified it from barium using its radioactivity. They named it from the Latin word radius (meaning "ray") because the strength of its radioactivity was more than a million times that of uranium.

Because of their metallic properties and low mass, Be and Mg are used to form lightweight alloys for structural purposes. Ca sees less industrial use, although the phosphate is sometimes utilized in fertilizers. Sr and Ba have no significant industrial applications. Both Be and Ra are used in various devices, the former because it is quite transparent to x-rays and the latter because it is a ready source of both α - and γ -radiation. Mg and Ca are essential to all living systems for many reasons; the other alkali earths have no known biological roles.

see also Beryllium; Cesium; Curie, Marie Sklodowska; Davy, Humphry; Francium; Magnesium; Potassium; Rubidium; WÖhler, Friedrich.

Michael E. Maguire

Bibliography

Nechaev, I.; Jenkins, G. W.; and Van Loon, Borin (1997). Chemical Elements: The Exciting Story of Their Discovery and of the Great Scientists Who Found Them. Jersey City, NJ: Parkwest Publications.

Rossotti, Hazel (1998). Diverse Atoms: Profiles of the Chemical Elements. New York: Oxford University Press.

Internet Resource

Winter, Mark (2003). WebElements Periodic Table, Scholar Edition. WebElementsLtd. Additional information available from <http://www.webelements.com>.

alkaline rock Igneous rock containing a relatively high concentration of the alkali (lithium, sodium, potassium, rubidium, caesium, and francium) and alkaline earth metals (magnesium, calcium, strontium, barium, and radium). Both silica-saturated and silica-undersaturated varieties exist, expressed in the presence of alkali feldspars and feldspathoids respectively. Alkali ferromagnesian minerals are usually present, and their identity depends on the composition of the rock. Igneous rocks of the alkaline suite span the composition range from basic to acid, and may be intrusive or extrusive.

alkaline rock An igneous rock that contains a relatively high concentration of the alkali (lithium, sodium, potassium, rubidium, caesium, and francium) and alkaline earth metals (magnesium, calcium, strontium, barium, and radium). Both silica-saturated and silica-under-saturated varieties exist, expressed in the presence of alkali feldspars and feldspathoids respectively. Alkali ferromagnesian minerals are usually present, and their identity depends on the composition of the rock. Compare acid rock.

alkaline-earth metals metals constituting Group 2 of the periodic table . Generally, they are softer than most other metals, react readily with water (especially when heated), and are powerful reducing agents, but they are exceeded in each of these properties by the corresponding alkali metal. They form divalent compounds. In order of increasing atomic number the alkaline-earth metals are beryllium , magnesium , calcium , strontium , barium , and radium .

alkaline-earth metals Bivalent metals forming Group II of the periodic table: beryllium, magnesium, calcium, strontium, barium, and radium. They are all light, soft and highly reactive. All, except beryllium and magnesium, react with cold water to form hydroxides (though magnesium reacts with hot water). Radium is important for its radioactive properties.

alkaline earths , oxides of the alkaline-earth metals , especially of calcium, strontium, and barium. They are not readily soluble in water and form solutions less basic than those of alkalies.