Boron

Boron in Periodic Table

Boron (B), atomic number 5, is the first chemical element of Group 13 or Group IIIA in the periodic table has the smallest size and highest electronegativity, the compounds are essential to plant growth and wide industrial application of ancient civilization. The small size, high ionization energy, and moderate electronegativity (close to carbon and hydrogen) explain the formation of many exciting and unusual covalent bond or compound of boron in chemistry. The natural boron contains two isotopes like ¹⁰B or B-10 (19.6 percent) and ¹¹B or B-11 (80.4 percent). Boron in the Group 13 of the periodic table member occurs in a small amount due to its nucleus being disintegrated by natural bombardment reactions but aluminum is very abundant, occupies the third position after oxygen and silicon.

Properties and Occurrence

Pure boron is a high melting solid diamagnetic substance (melting point = 2180 °C) which be either crystalline (black) or amorphous (brown), uses as a semiconductor conducts electricity at high temperature like a metal. Crystalline boron exists in several allotropic forms which are structurally very complex, build up by B₁₂ icosahedra. Different types of packing on icosahedra in-unit crystal lattice form different types of the structural formula of boron, the simplest form is α-rhombohedral.

The abundance of boron in crustal rocks (9 ppm) of our earth universe is very low which is less than that of lithium (18 ppm) or lead (13.1 ppm). The vast deposits of boron minerals like borate or borosilicates are occurred in the regions of California, Turkey, Russia, and Argentina by former volcanic activities and water from hot springs. The principal minerals of boron are colemanite, Ca₂[B₃O₄(OH)₃]₂, 2H₂O; kernite Na₂[B₄O₅(OH)₄], 2H₂O; and borax, Na₂[B₄O₅(OH)₄], 8H₂O. In India, Tibet, and Sri Lanka, borax occurs as a precipitate from hot springs.

History, Isolation, and Uses

In history, the different civilizations of the world, the compound of boron borax uses as a flux, or prepared glazes and hard glass. The element itself was isolated in the nineteenth century by Davy, Gay Lussac, and Thenard by electrolysis of moist boric acid. Moissan in 1892 prepared a rather pure specimen of boron (95 percent) by heating B₂O₃ with magnesium metal and the name proposed by Devay to illustrate its source and similarities to carbon. The crystalline form of high purity element is now obtained by reduction of volatile boron compounds (BCl₃ or better BBr₃) with hydrogen on heated titanium wire. However, the process can operate to form the element on a kilogram scale. Below 1000 °C, amorphous boron is obtained, between 1000 to 1200 °C α and β-rhombohedral form is obtained and above 1200 °C tetragonal crystal is formed.

In large quantities, the amorphous boron is obtained by reduction of B₂O₃ with magnesium (Mg) or other electropositive metals at high temperatures (B₂O₃ + 3Mg → 2B + 3MgO). The reaction is exothermic with the free energy at 298K = -515 kJ. The unreacted substances and MgO can be removed by washing the substance with dilute hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions. The 95 percent pure boron powdered is cheaply obtained by electrolytic reduction of KBF₄ in molten KCl or KF at 800 °C.

Boron and its compounds are extensively used in filaments and fiber composites used in reinforcement materials (space shuttles and aircraft). Boron carbides are used as abrasive for polishing or grinding, the carbides and metal borides are extensively used in neutron shielding and control rods for nuclear power plants. The isotope of boron (¹⁰B) has a high absorption cross-section for high energy neutrons (10⁴ to 10⁶ eV) is a more beneficial substance in B-10 neutron capture therapy of brain tumors. Metal borides (TiB₂, ZrB₂) have found much technical application. Due to the physical and chemical properties like high melting point, hard nature, and chemically inertness of metal borides (compound of boron), metal borides are used as a coating on turbine blades, rocket nozzles, and high-temperature reaction vessels. Boric acid, B₂O₃, and borax are used in medicine, in making borosilicate glass, enamels, and fire retardant.

Chemical Reactivity of Boron

The crystalline elemental form of boron is chemically very inert and not affected by acids or oxidizing agents but fused sodium hydroxide (NaOH) at 500 °C attacks to form NaBO₂ and hydrogen gas. The amorphous form is more reactive than the crystalline form. Therefore, the amorphous elements of Group 13 (B, Al, Ga, In) burns in the air to form trioxides and nitrides. It dissolves in oxidizing acid-like nitric acid (HNO₃) or sulfuric acid (H₂SO₄) but unreacted with hydrochloric acid (HCl). Only boron and aluminum combine directly with nitrogen when heated to form BN and AlN but GaN and InN may be obtained indirectly by heating the metals with ammonia.

Boron Compounds

The electron configuration of boron is 1s² 2s² 2p¹, with the two electrons in s-orbitals and one electron in p-orbital suggest the monovalent state but it never exhibits +1 oxidation number, only shows +3 state of the chemical element. The chemistry of the first member of the boron family is naturally influenced by the small size and high ionization energy and the compounds will be essentially covalent chemical bonding with the oxidation state III. The formation of B⁺³ ion appears unlikely for two main reasons, the ionization energy for the formation of B⁺³ ion would be very large, and small ion has highly polarizing properties. Due to the presence of three bond pairs, the compounds of boron function as a Lewis acid by accepting electron pairs from the Lewis base.

The electron deficiency defines the unique characteristics of boron to form Pi-bonds with elements like chlorine, fluorine, etc. In BH₃, BCl_3, or BF₃, the element attains noble gas configuration through dimerization of the molecule. Due to its small size, the boron atom can readily form a stable lattice with metal atoms (interstitial alloy), chain species, and three-dimension networks. Depending upon the individual structure and chemical reactivity, the principal boron compounds may be classified into, hydrides and their derivatives (carboranes and polyhedral borane metal complexes), halides, oxygen compounds including polyborates, borosilicates, metal borides, nitrogen, and organoboron compounds in learning chemistry.