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ChemWiki: The Dynamic Chemistry E-textbook > Inorganic Chemistry > Descriptive Chemistry > p-Block Elements > Group 13: The Boron Family > Chemistry of Boron

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Chemistry of Boron

Boron is the fifth element of the periodic table, located in Group 13. It is classified as a metalloid due it its ambiguous properties that reflect a combination of both metals and nonmetals.

Introduction

General Properties:

 

Atomic Mass 10.811 g/mol
Electronic Configuration [He]2s2 2p1
Melting Point 2349 K
Boiling Point 4200 K
Heat of Fusion 50.2 kJ/mol
Heat of Vaporization 480 kJ/mol
Specific Heat Capacity 11.087 J/mol·K
Oxidation States +4, +3, +2, +1
Magnetic Ordering diamagnetic
Electronegativity 2.04
Atomic Radius 90 pm
Stable Isotopes 10B, 11B

 

→Boron is the only element in its group that is not a metal. It has properties that lie between metals and non-metals (semimetals). For example Boron is a semiconductor unlike the rest of the group 13 elements. Chemically, it is closer to aluminum than any of the other group 13 elements.

History

Boron was first discovered by Joseph-Louis Gay-Lussac and Louis-Jaques Thenard, and independently by Humphry Davy in the year 1808. These chemists isolated Boron by combining boric acid with potassium. Today, there are many ways of obtaining Boron but the most common way is by heating borax (a compound of sodium and boron) with calcium. 

Boron and its Compounds

Many boron compounds are electron-deficient, meaning that they lack an octet of electrons around the central boron atom. This deficiency is what accounts for boron being a strong Lewis acid, in that it can accept protons (H+ ions) in solution.  Boron-hydrogen compounds are referred to as boron hydrides, or boranes.

Boranes

In the molecule BH3, each of the 3 hydrogen atoms is bonded to the central boron atom. The boron atom has only six electrons in its outer shell, leading to an electron deficiency.

Diborane:

                H         H

                 I           I               

         H -  B    ?   B -  H

 

                 I           I

                H         H

 

This molecule has 12 valence shell electrons; 3 each from the B atoms, and 1 each from the six H atoms. To make this structure follow the rules required to draw any lewis structure model, then it must have 14 valence shell electrons; however it does not. According to this figure, the two B atoms and four H atoms lie in the same plane (sp3- perpendicular to the plane of the page). In these four bonds 8 electrons are involved. Four electrons bond the remaining H atoms to the two B atoms and the B atoms together. This is done when the two H atoms simultaneously bond to the two B atoms. This creates what is called an atom "bridge" because there are two electrons shared among three atoms. These bonds are also called three-center two-electron bonds. The bond between the H and the B atoms can be rationalized using molecular orbital theory.

Other Boron Compounds

Although boron compounds are widely distributed in Earth's crust, a few concentrated ores are located in Italy, Russia, Tibet, Turkey, and California. Borax is the most common ore found, and it can be turned into a variety of boron compounds. When a solution of borax and hydrogen peroxide is crystallized, sodium perborate (NaBO3 * 4 H2O) is formed. Sodium perborate is used in color-safe bleaches. The key to the bleaching ability of this compound is the presence of its two peroxo groups that bridge the boron atoms together. Another compound that other boron compounds can be synthesized from is boric acid (B(OH)3). When mixed with water, the weakly acidic and electron deficient boric acid accepts an OH- ion from water and forms the complex ion [B(OH)4]-.

Borate salts produce basic solutions that are used in cleaning agents. Boric acid is also used as an insecticide to kill roaches, and as an antiseptic in eyewash solutions. Other boron compounds are used in a variety of things, for example: adhesives, cement, disinfectants, fertilizers, fire retardants, glass, herbicides, metallurgical fluxes, and textile bleaches and dye.

References

  1. Adair, Rick, ed. Boron. The Rosen Group Inc., 2007. Print.
  2. Hasan, Heather. The Boron Elements: Boron, Aluminum, Gallium, Indium, Thallium. Rosen Group, 2009. Print.

Problems

  1. What is the electronic configuration of boron?
  2. What accounts for the formation of boron hydrides?
  3. What are some uses of boron compounds?
  4. Draw B4H10.
  5. What is the molecular orbital theory and how is it used to rationalize the bonds in boron hydrides?

Answers

  1. [He]2s2 2p1
  2. Boron is highly electronegative, and wants to form compounds with hydrogen atoms.
  3. Adhesives, cement, disinfectants, fertilizers, etc.
  4. b4h10.png
  5. The molecular orbital theory treats compounds not as having individual bonds between atoms, but as sharing electrons with multiple atoms through their orbitals.  In this way, hydrogen atoms are "bonded" between 2 other atoms at a time, in that a pair of electrons is shared between 3 atoms at once.

Contributors

  • Forogh Rahim (UCD)

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Last Modified
13:00, 6 Feb 2014

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