176 | The History and Use of Our Earth’s Chemical Elements Properties
176 | The History and Use of Our Earth’s Chemical Elements Properties
Boron has only three electrons in its outer shell, which makes it more metal than nonmetal. Nonmetals have four or more electrons in their valence shell. Even so, boron is somewhat related to metalloids and also to nonmetals in period 2.
It is never found in its free, pure form in nature. Although less reactive than the metals with fewer electrons in their outer orbits, boron is usually compounded with oxygen and sodium, along with water, and in this compound, it is referred to as borax. It is also found as
a hard, brittle, dark-brown substance with a metallic luster, as an amorphous powder, or as shiny-black crystals. Its melting point is 2,079°C, its boiling point is 2,550°C, and its density is 2.37 g/cm 3 .
Characteristics Boron is a semimetal, sometimes classed as a metallic or metalloid or even as a nonmetal.
It resembles carbon more closely than aluminum, the latter of which is located just below boron in group 13. Although it is extremely hard in its purified form—almost as hard as diamonds—it is more brittle than diamonds, thus limiting its usefulness. It is an excellent conductor of electricity at high temperatures, but acts as an insulator at lower temperatures. It is less reactive than the elements below it in group 13
AbundanceandSource Boron is the 38th most abundant element on Earth. It makes up about 0.001% of the
Earth’s crust, or 10 parts per million, which is about the same abundance as lead. It is not found as a free element in nature but rather in the mineral borax, which is a compound of hydrated sodium, hydrogen, and water. Borax is found in salty lakes, dry lake-beds, or alkali soils. Other naturally occurring compounds are either red crystalline or less dense, dark-brown or black powder.
Boron is also found in kernite, colemanite, and ulexite ores, and is mined in many coun- tries, including the western United States.
History While experimenting with electrodes for batteries, Sir Humphry Davy (1778–1829) dis-
covered that some of these metal-containing compounds would break down when a current was passed through water containing a solution (electrolyte) of the compound. The metal was deposited on one electrode, and hydrogen gas was released from the water. He used this method to isolate barium, strontium, calcium, magnesium, and boron. Davy is given credit for discovering boron in 1808.
In 1808 two French chemists, Joseph Louis Gay-Lussac (1778–1850) and Louis-Jacques Thenard (1777–1857), experimented along the same lines as Davy and should also be given some credit for the discovery of these elements. The Frenchmen named the new element “bore,” and Davy called it “boracium.”
Alfred Stock (1876–1946) studied the hydrides of some of these metal-like elements. A hydride occurs when hydrogen gains (or shares) an electron rather than losing its single elec- tron when it combines with metals or metallic-like elements. Stock spent years experimenting
with boron hydrides (B 6 H 6 and BH 3 ), which were used as hydrogen-based rocket fuels power-
177 CommonUses
Guide to the Elements |
Even though boron has a very simple atom with just five protons in its nucleus and only three valence electrons, it has proven to be somewhat bewildering and continues to intrigue chemists as a more-or-less exotic element. Even so, boron has found many uses and has become an important industrial chemical.
Borax is used as a cleaning agent and water softener that removes ions of elements such as magnesium and calcium that cause hard water. When these “hard” water elements are mixed with soap, they prevent soap from sudsing and form a scum or residue that is deposited on hard surfaces. Borax can eliminate this residue ring by replacing the Mg ++ and Ca ++ ions with the more soluable Na + and K + ions. Borax is the third most important boron compound.
A common but important compound of boron is boric acid (H 3 BO 3 ), which is made by heating borax with an acid (either HCl or H 2 SO 4 ). Boric acid is weak and can be used as eyewash. More importantly, it is used to manufacture heat-resistant borosilicate glass that is known by the trade name Pyrex. Pyrex is commonly used for baking utensils, so that a drastic change in temperature will not damage the glass. Pyrex is an example of one of many products developed by NASA’s space program that led to everyday practical use. Boric acid is the second most important compound of the element boron. Sodium borate pentahydrate
(Na 2 B 4 O 7 • 5H 2 O) is the most important boron compound; it is used to make fiberglass insulation.
A hydride of boron that is combined with hydrogen is an effective “booster” for rocket fuel in spacecrafts. Boron is used as an alloy metal, and when combined with other metals, it imparts excep- tional strength to those metals at high temperatures. It is an excellent neutron absorber used to “capture” neutrons in nuclear reactors to pre- vent a runaway fission reaction. As the boron rods are lowered into the reactor, they control the rate of fission by absorbing excess neutrons. Boron is also used as an oxygen absorber in the production of copper and other metals,
Boron finds uses in the cosmetics industry (talc powder), in soaps and adhesives, and as an environmentally safe insecticide.
A small amount of boron is added as a “dope” to silicon transistor chips to facilitate or impede the flow of current over the chip. Boron has just three valence electrons; silicon atoms have four. This dearth of one electron in boron’s outer shell allows it to act as a positive “hole” in the silicon chip that can be “filled” or left vacant, thus acting as a type of switch in transis- tors. Many of today’s electronic devices depend on these types of doped-silicon semiconduc- tors and transistors.
Boron is also used to manufacture borosilicate glass and to form enamels that provide a protective coating for steel. It is also used as medication for relief of the symptoms of arthri- tis.
Due to boron’s unique structure and chemical properties, there are still more unusual compounds to be explored.
ExamplesofCompounds Boron’s +3 oxidation state permits it to join ions with oxidation states of –1, –2, and –3
as follows:
178 | The History and Use of Our Earth’s Chemical Elements
Boron fluorides: B 1- + 3F → BCl
Boron oxide: 2B 2- + 3O →B
Boron nitrate (B 3- +N → BN) is a white powder used to line high-temperatures furnaces, to make heating crucibles, for electrical and chemical equipment, for heat shields on spacecraft
nosecones, and to make high-strength fabrics. Boron-10, a stable isotope of boron, is used to absorb slow neutrons in nuclear reactors. It produces high-energy alpha particles (helium nuclei) during this process. Boron carbide (B 4 C) is a hard, black crystal that is used as an abrasive powder and as an additive to strengthen composite parts in aircraft. Boric acid (boracic acid; H 3 BO 3 ) is used for the manufacture of glass, welding, mattress batting, cotton textiles, and a weak eyewash solution. Refined borax (Na 2 B 4 O 7 ) is an additive in laundry products such as soaps and water-soften- ing compounds. Also used for cosmetics, body powders, and the manufacture of paper and leather. Borax is an environmentally safe natural herbicide and insecticide.
Hazards Powdered or fine dust of elemental boron is explosive in air and toxic if inhaled. Several of
the compounds of boron are very toxic if ingested or if they come in contact with the skin. This is particularly true of the boron compounds used for strong insecticides and herbicides.
ALUMINUM SYMBOL:Al PERIOD:3 GROUP:13(IIIA) ATOMICNO:13
ATOMICMASS:26.981538amu VALENCE:3 OXIDATIONSTATE:+3 NATURALSTATE:Solid ORIGINOFNAME:FromtheLatinwordalumen,oraluminis,meaning“alum,”whichisa bittertastingformofaluminumsulfateoraluminumpotassiumsulfate. ISOTOPES:Thereare23isotopesofaluminum,andonlyoneoftheseisstable.Thesingle stableisotope,Al-27,accountsfor100%oftheelement’sabundanceintheEarth’s crust.Alltheotherisotopesareradioactivewithhalf-livesrangingfromafewnanosec- ondsto7.17×10 +15 years.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=3
s2,p1
179 Properties
Guide to the Elements |
Pure metallic aluminum is not found in nature. It is found as a part of compounds, especially compounded with oxygen as in aluminum oxide (Al 2 O 3 ). In its purified form, alu- minum is a bluish-white metal that has excellent qualities of malleability and ductility. Pure aluminum is much too soft for construction or other purposes. However, adding as little as 1% each of silicon and iron will make aluminum harder and give it strength.
Its melting point is 660.323°C, its boiling point is 2,519°C, and its density is 2.699 g/ cm 3 .
Characteristics Alloys of aluminum are light and strong and can easily be formed into many shapes—that
is, it can be extruded, rolled, pounded, cast, and welded. It is a good conductor of electricity and heat. Aluminum wires are only about 65% as efficient in conducting electricity as are copper wires, but aluminum wires are significantly lighter in weight and less expensive than copper wires. Even so, aluminum wiring is not used in homes because of its high electrical resistance, which can build up heat and may cause fires.
Aluminum reacts with acids and strong alkali solutions. Once aluminum is cut, the fresh surface begins to oxidize and form a thin outer coating of aluminum oxide that protects the metal from further corrosion. This is one reason aluminum cans should not be discarded in the environment. Aluminum cans last for many centuries (though not forever) because atmo- spheric gases and soil acids and alkalis react slowly with it. This is also the reason aluminum is not found as a metal in its natural state.
AbundanceandSource Aluminum is the third most abundant element found in the Earth’s crust. It is found in
concentrations of 83,200 ppm (parts-per-million) in the crust. Only the nonmetals oxygen and silicon are found in greater abundance. Aluminum oxide (Al 2 O 3 ) is the fourth most abundant compound found on Earth, with a weight of 69,900 ppm. Another alum-type compound is potassium aluminum sulfate [KAl(SO 4 ) 2 •12H 2 O]. Although aluminum is not found in its free metallic state, it is the most widely distributed metal (in compound form) on Earth. Aluminum is also the most abundant element found on the moon.
Almost all rocks contain some aluminum in the form of aluminum silicate minerals found in clays, feldspars, and micas. Today, bauxite is the major ore for the source of aluminum metal. Bauxite was formed eons ago by the natural chemical reaction of water, which then formed aluminum hydroxides. In addition to the United States, Jamaica and other Caribbean islands are the major sources of bauxite. Bauxite deposits are found in many countries, but not all are of high concentration.
History Beginning in the 1700s and beyond, scientists suspected that an unknown metal existed in
alum. Alum is found in the form of several compounds—for example, aluminum ammonium sulfate [AlNH 4 ISO 4 ) 2 •12H 2 O], aluminum potassium sulfate [AlK(SO 4 ) 2 •12H 2 O], or alumi- num sulfate [Al 2 (SO 4 ) 3 ]. The scientists’ problem was that they had no techniques or knowledge of how to extract the metal from its ore until 1825 when the Danish chemist Hans Christian
180 | The History and Use of Our Earth’s Chemical Elements sium. However, it was not enough for practical use. Napoleon considered it a precious metal.
Later, a German scientist, Friederich Wohler (1800–1882), found a way to extract enough aluminum to analyze its chemical and physical properties. In the latter part of the nineteenth century, a French chemist, Henri Etienne Sainte-Claire Deville (1818–1881), produced the first commercial aluminum, which reduced the price from about $1,200 per pound to about $40 a pound. This was still too expensive for commercial use, but aluminum “silverware” and other utensils were all the rage and used by royalty during this time in history.
This picture changed in the 1886 when an American chemist, Charles Martin Hall (1863– 1914), and a French chemist, Paul Louis-Toussaint Heroult (1863–1914), both discovered, at about the same time, a new process for extracting aluminum from molten aluminum oxide by electrolysis. (It might be noted that both discoverers have the same birth and death dates as well as the same date of discovery.) Hall was inspired by his teacher to find a way to inexpensively pro- duce aluminum metal. He wired together numerous “wet cells” to form a “battery” that produced enough electricity to separate the aluminum from the melted aluminum oxide (mixed with the minerals cryolyte or fluorite), by the process known as electrolysis. Hall formed the Pittsburgh Reduction Co., which is now known as the Aluminum Company of America, or Alcoa. His com- pany produced so much aluminum that the price dropped to about sixty cents per kilogram.
A few years later, an Austrian chemist, Karl Joseph Bayer, refined Hall’s process, and it is now called the Hall-Heroult or Bayer process, which is the method used today for obtaining aluminum at very reasonable prices
CommonUses Aluminum is a very versatile metal with many uses in today’s economy, the most common of
which are in construction, in the aviation-space industries, and in the home and automobile indus- tries. Its natural softness is overcome by alloying it with small amounts of copper or magnesium that greatly increase its strength. It is used to make cans for food and drinks, in pyrotechnics, for protec- tive coatings, to resist corrosion, to manufacture die-cast auto engine blocks and parts, for home cooking utensils and foil, for incendiary bombs, and for all types of alloys with other metals.
Aluminum does not conduct electricity as well as copper, but because it is much lighter in weight, it is used for transmission lines, though not in household wiring. A thin coating of aluminum is spread on glass to make noncorroding mirrors. Pure oxide crystals of aluminum are known as corundum, which is a hard, white crystal and one of the hardest substances known. Corundum finds many uses in industry as an abrasive for sandpaper and grinding wheels. This material also resists heat and is used for lining high-temperature ovens, to form the white insulating part of spark plugs, and to form a protective coating on many electronic devices such a transistors.
Aluminum oxide is used to make synthetic rubies and sapphires for lasers beams. It has many pharmaceutical uses, including ointments, toothpaste, deodorants, and shaving creams. Aluminum scrap is one of the salvaged and recycled metals that is less expensive to reuse than it is to extract the metal from its ore. In other words, it takes much less electricity to melt scrap aluminum than it does to extract aluminum from bauxite.
ExamplesofCompounds Several examples of compounds in aluminum’s oxidation state of +3 follow:
Aluminum chloride: Al 3+ + 3Cl 1- → AlCl 3. Aluminum chloride is a crystal that vaporizes in air
181 Aluminum fluoride: 2Al 3+ + 3F 2- → Al 2 F 3 . Aluminum fluoride is used to produce low-
Guide to the Elements |
melting aluminum metal, as a flux in ceramic glazes and white enamels, and as a catalyst in chemical reactions.
Aluminum oxide: 2Al 3+ + 3O 2- → Al 2 O 3 is known as the mineral bauxite. Its main use is for the production of aluminum metal by electrolysis. It is also used in many other chemical reactions. Aluminum sulfate: Al 2 O 3 + 3H 2 SO 4 → Al 2 (SO 4 ) 3 + 3H 2 O. This reaction treats bauxite with sulfuric acid, resulting in aluminum sulfate plus water. Aluminum sulfate is also known as alum, which acts as an astringent to stop a minor flow of blood or dry up blisters (potassium aluminum sulfate is also an alum).
Aluminum alloys are not really compounds, but rather mixtures of other metals with alu- minum to produce stronger metal. Some of the metals used as alloy metals with aluminum are copper, manganese, silicon, magnesium, zinc, chromium, zirconium, vanadium, lead, and bismuth. One of the most important alloys is known as duralumin. It is a heat-treated mixture of aluminum, copper, magnesium, and manganese. It is very strong, lightweight, and noncor- rosive, making it useful in the aerospace industries.
Hazards Aluminum dust and fine powder are highly explosive and can spontaneously burst into
flames in air. When treated with acids, aluminum chips and coarse powder release hydrogen. The heat from the chemical reaction can then cause the hydrogen to burn or explode. Pure aluminum foil or sheet metal can burn in air when exposed to a hot enough flame. Fumes from aluminum welding are toxic if inhaled.
GALLIUM SYMBOL:Ga PERIOD:4 GROUP:13(IIIA) ATOMICNO:31
ATOMICMASS:69.723amu VALENCE:2and3 OXIDATIONSTATE:+3 NATURALSTATE:Solid ORIGINOFNAME:LatinwordGallia,meaning“Gaul,”anearlynameforFrance. ISOTOPES:Thereare33isotopesofgallium,twoofwhicharestable.TheyareGa-69,which
makesup60.108%oftheelement’spresenceintheEarth’scrust,andGa-71,whichcon- tributes39.892%ofthegalliumfoundintheEarth’scrust.Alltheother31isotopesare radioactivewithhalf-livesrangingfromafewnanosecondstoabout15hours.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10