Group 3A

 

Group 3A (or IIIA) of the periodic table includes the metalloid boron (B), as well as the metals aluminum (Al), gallium (Ga), indium (In), and thallium (Tl).  Boron forms mostly covalent bonds, while the other elements in Group 3A form mostly ionic bonds.

The Group 3A metals have three valence electrons in their highest-energy orbitals (ns²p¹). They have higher ionization energies than the Group 1A and 2A elements, and are ionized to form a 3+ charges.

The Group 3A metals are silvery in appearance, and like all metals are good conductors of electricity.  They are relatively soft metals, with lower melting points than many of the Group 2A metals.  Aluminum melts at 660ºC, while gallium melts at 29.8ºC.  (A chunk of gallium would literally melt in your hand — that it, if you have the usual body temperature of 37ºC.)

 

Boron (B, Z=5).

Boron is found as shiny black crystals, or as an amorphous brown powder.  The name of the element is derived from buraq, the Arabic word for borax.  It is found in the Earth's crust at a concentration of 10 ppm, making it the 38th most abundant element.  It is found in the ores kernite [hydrated sodium borate hydroxide, Na₂B₄O₆(OH)₂·3H₂O], ulexite [hydrated sodium calcium borate hydroxide, NaCaB₅O₉·8H₂O], and colemanite [a hydrated calcium borate, Ca₂B₆O₁₁·5H₂O].

Boron is classified as a metalloid, having properties of both metals and nonmetals:  it and conducts electricity at high temperatures; but at room temperature, is it an insulator.  Many boron salts emit a green color when heated.

Boron is most commonly found in the form of boron-oxygen salts called borates, such as borax or sodium borate, Na₂B₄O₇·10H₂O (the number of coordinated water molecules can vary, depending on the source), and boric acid, H₃BO₃.  Borate salts are used in the refining of metals, in the manufacture of glass, in fertilizers (boron is necessary for plant growth), and in detergents.  Pyrex glassware is a borosilicate glass introduced by Corning Glass Works in 1915; it is widely used in laboratories because it does not expand on heating as much as ordinary glass, and it is much more resistant to breaking.  Boric acid is used in many insecticides.  Boron can also form extremely hard materials with nitrogen and carbon; boron nitride, BN, is similar in strength and appearance to diamond, and is used as an abrasive.

 

Aluminum (Al, Z=13).

Aluminum is a relatively soft, silvery, malleable metal.  The name of the element is derived from the Latin word for alum, alumen.  In Europe, the name is spelled "aluminium," while in the United States, it is spelled "aluminum."  The Earth's crust is 8.3% aluminum by weight, making it the third most abundant element, and the most abundant metal.  It is found in the ores boehmite [AlO(OH)], gibbsite [aluminum hydroxide, Al(OH)3], and bauxite [a mixture of boehmite and gibbsite, often abbreviated as Al₂O₃].

Despite its ubiquity, it was a valuable substance for a long time because of the extreme difficulty of obtaining it in its elemental, metallic form.  Aluminum can be obtained from bauxite ore by heating, but this requires such high temperatures that this is a very inefficient — and expensive — process.  Aluminum is easier to obtain through electrolytic process, such as the Hall-Heroult process (invented by Charles Hall and Paul Heroult independently of each other in 1886), in which bauxite ore is dissolved in molten cryolite (Na₃AlF₆) at a temperature of about 1000ºC and zapped with electricity, which reduces the Al³⁺ ions to neutral Al atoms.  The molten aluminum that forms under these conditions pools up at the bottom of the electrolytic cell, where it can be drained off and cast into large blocks, which can then be rolled out to whatever thickness is desired.  This process still requires large amounts of electricity because of aluminum's 3+ charge (about 5% of the electricity generated in the United States goes toward aluminum production), making the recycling of aluminum economically desirable.

Aluminum oxidizes in air to form aluminum oxide, Al₂O₃.  Unlike oxidized iron (rust, Fe₂O₃), the oxidized aluminum does not flake off:  it forms a thin, hard protective layer on the outside of the aluminum, which protects the rest of the metal from further oxidation.

Aluminum is strong and lightweight, and is used in many applications ranging from construction to cookware.  Aluminum is often alloyed with other metals to improve its strength and mechanical properties.  One particularly useful alloy is duralumin, an alloy of aluminum with about 4% copper, and small amounts of magnesium, manganese, iron, and silicon.  Aluminum and its alloys are used in aircraft and automobile frames, rockets, window frames, doors, siding, kitchenware, packaging (aluminum cans, aluminum foil), electrical wiring (although aluminum has a higher resistance than copper, it is cheaper), as a silvering agent in paints (usually in powdered form), in heat sinks, and many other uses.

Aluminum oxides are the basis for many gemstones, such as sapphires and rubies:  the blue color of sapphires comes from trace amounts of cobalt in the aluminum oxide framework, while the red color of rubies comes from traces of chromium.

Potassium aluminum sulfate, KAl(SO₄)₂, also known as alum, has been used for centuries as a mordant in dyeing, and in medicines as an astringent (a substance which causes tissues and blood vessels to contract) to stop bleeding.  (In the form of styptic pencils, alum was an essential ingredient in shaving until the development of safety razors.)

 

Gallium (Ga, Z=31).

Gallium is a soft, silvery-white metal with a low melting point (29.8ºC); unlike most metals, it expands when cooled.  The name of the element is derived from Gallia, the Latin name for France (although it has been suggested that its discoverer, Paul-Emile Lecoq de Boisbaudran, snuck his own name into the periodic table, since the name "Lecoq", which means "rooster" in French, translates into Latin as gallus).  It is found in the Earth's crust at a concentration of 18 ppm, making it the 34th most abundant element.  It is found in trace amounts in n bauxite, coal, diaspore, germanite, and sphalerite.

The existence of gallium was predicted by Dimitri Mendeleev in 1869 from a blank space in his periodic table beneath aluminum; before it was actually found, the hypothetical element was referred to as "eka-aluminum" (eka is the Sanskrit word for "one," meaning that it was one space away from aluminum on the periodic table).  When gallium was discovered in 1875, its physical and chemical properties matched many of those predicted by Mendeleev from its position in his table.

Gallium arsenide, GaAs, is a semiconductor, and is used in making LEDs, transistors, and diodes used in lasers.  It is also heavily used in computer circuitry.

 

Indium (In, Z=49).

Indium is a soft, silvery metal.  The name of the element is derived from the Latin word for indigo, indicum, because of the bright violet line of its atomic spectrum.  It is found in the Earth's crust at a concentration of 0.1 ppm, making it the 69th most abundant element.  It is found in the rare mineral indite [FeIn₂S₄], and in trace emounts in some zinc ores; it has also been found as the free metal.

Indium is used in alloys with other metals to lower their melting points, and is also used in some dental appliances.

 

Thallium (Tl, Z=81).

Thallium is a soft, gray metal; unlike the other Group 3A metal, its oxides flake away from the rest of the metal, exposing a fresh surface for more oxidation to take place on.  The name of the element is derived from the Greek word thallos, which means "green twig"; this name was given because of the bright green emission lines in the element's atomic spectrum.  It is found in the Earth's crust at a concentration of 0.6 ppm, making it the 59th most abundant element.  It is found in the ores crookesite [Cu7TlSe4], hutchinsonite [TlPbAs₅S₉], and larandite [TlAsS2], but is more frequently obtained as a byproduct of the refining of lead and zinc..

Thallium is toxic, and the symptoms of thallium poisoning are often mistaken for other things, making thallium a popular device in both fictional murders (see Agatha Christie's novel The Pale Horse, 1961) and real murders (see John Emsley's The Elements of Murder: A History of Poison, 2005).  Thallium(I) sulfate, Tl₂SO₄, was used as a pesticide, but has been banned in the United States since 1975.

 

 

References

John Emsley, The Elements, 3rd edition.  Oxford:  Clarendon Press, 1998.

John Emsley, Nature's Building Blocks:  An A-Z Guide to the Elements.  Oxford:  Oxford University Press, 2001.

David L. Heiserman, Exploring Chemical Elements and their Compounds.  New York:  TAB Books, 1992.