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Boron

History Of Boron:

  History and discovery of Boron is very interesting indeed. While chemical compounds containing Boron were around and used, the element Boron was not discovered by itself till 1800.It was only after that, was element Boron discovered.

Discovery of Boron and the part played by Sir Davy in isolating it as an element

  Rhazes, a Persian alchemist, who lived in the 9th and 10th centuries, made the earliest record of compounds with boron. Rhazes grouped minerals into six classes. One of them was boraces, which included the powder borax as we know it today. Alchemists studied and analyzed matter and its structure back then. Borax was popular among craftsmen. It has the property to reduce melting points of components used to make glass. It was also used to melt and purify metals from their ores. Then in 1808, an English chemist called Humphrey Davy found a way to isolate highly reactive metals like sodium or potassium from their compounds. He used the same to isolate boron. Seeing this, emperor Napoleon Bonaparte arranged for two French chemists to do the same. Their method was successful only in manufacturing impure boron. Yet, the credit for discovery of the new metal Boron went to them. It was noted later that Dave's boron was the purest.

Boron family:

  Boron, Aluminium (Al), Gallium ( Ga), Indium (In) and thallium (Tl) constitute the elements of group III of the periodic table . The general outer shell configuration of these elements in ns2 np1 . They exhibit similar properties because of similar outer shell electronic configuration . The penultimate shell contains two electrons in boron and 8 electrons in aluminium, the other elements contain 18 electrons.  Because  of this difference boron differs from aluminium in some properties and these two elements  (B, Al) differ from other elements of this group in quite  a few other properties .



Element             Electronic configuration
Boron                [He] 2S2 2p1
Aluminium          [Ne] 3s2 3p1
Gallium              [Ar] 3d10 4s2 4p1
Indium               [kr] 4d10 5s2 5p1
Thalium              [Xe] 4f14 5d10 6s2 6p1



Trends in chemical reactivity

 The chemistry of Boron is different in many ways from that of the other elements in the group. Boron shows completely the non-metallic nature. With metals only Boron forms borides while others almost do not react. Boron forms an oxide and a nitride when burnt in air.

                            B    +    air          B2O3       +      BN.

           Boron compounds exhibit different properties from other elements compounds.

Nature of Compounds in Boron Family:

  The elements of this group III form mostly covalent compounds because of their high ionization energy .   While the compounds of boron are always covalent.  Boron and aluminium have similar properties due to their outer electronic configuration .  Both reacts with nitrogen forming nitrides which later react with water to give ammonia. Boron is non metal and a bad conductor of electricity , whereas aluminium is a metal and a good conductor of electricity.  Boron exhibits allotropy. It exists in crystalline and amorphous forms.

  Melting and Boiling points:  No regular trend is observed in the melting points of these elements due to the structural changes in the elements . The M.P is very high because it exists as large covalent polymer in solid and liquid states. The low melting point of gallium is due to its simple molecular structure.

Boiling point decrease regularly in boron family

Density and Oxidation States of Boron Family:
Density :

  Density increases from boron to thallium. Aluminium has lower density because of its large atomic size.

Oxidation state

  All the elements of boron family exhibit a common oxidation state of +3 because of their outer electronic configuration. Except B and Al all the other elements exhibit +1 oxidation state. The +1 oxidation state becomes high stable as we go down the group . Due to no much increase in atomic size and lower shielding effect of d and f electrons ns2 electrons are under great  influence of the nucleus and show reluctance towards participation in bond formation. This is called as inner pair effect .

  Boron exhibit -3 oxidation state in borides eg . Mg3B2

Important Boron Compounds

  Generally, Boron compounds are divided into 5 sub categories, which are as follows:
  * Boronates (Borane, Diborane, Decaborane, etc.)
  * Borates (Borate, Borax, Boric acid, Sodium perborate , etc.)
  * Borides (Boride, Calcium Hexaboride , uranium Boride , etc.)
  * Borohydrides (Borohydride, Lithium Borohydride, uranium borohydride, etc.)
  * Boron heterocycles (Borazines , Boroles , Borabenzenes,etc.)

  Under these 5 sub categories all important Boron compounds comes. And they are classifies like this due to their properties (chemical and Physical).

Uses of Important Compounds of Boron

  The most important compounds of Boron are as follows:
  * Sodium TetraBorate Decahydrate (Na2B4O7.10H2O) or it is commonly known as Borax. This compound of Boron is used for insulating fiberglass and sodium perborate bleach.
  * One of the other important compounds of Boron is Boric acid. It is important because it is widely used in textile industry, mainly in the manufacture of textile fiberglass and in cellulose insulation as a flame retardant.
  * Boron Nitride is a hard compound of boron used in electrical insulators possessing similar properties to that of graphite.
  * Boron-10 is used in nuclear reactors to control, detect neutrons and as a shield for nuclear radiations.
  * Many other Boron compounds are used in various applications like in leather tanning industries, cosmetics, photographic materials, soaps and cleaners and for high-energy fuel.

Physical properties of boron elements:

  Boron exists in two allotropic forms amorphous and crystalline boron. Boron is a non-metallic element and is a non-conductor of electricity.

Chemical properties of boron element:

1) Action of air:- It is unaffected by air at ordinary temperature but when heated in air to about 975K, it burns forming boron trioxide and a little boron nitride, BN
4B + 3O2  ---------------> 2B2O3
2B + N2 ------------------> 2BN

2) With acids: - Amorphous boron dissolves in hot concentrated sulphuric and in nitric acid to form boric acid.
B + 3HNO3 --------------->  H3BO3 + 3NO2
2B + 3H2SO4 ------------------------>  2H3BO3 + 3SO2.

3) With caustic alkali:- It dissolves in fused caustic alkali and forms boric acid.

4) As a reducing agent:- Boron is a powerful reducing agent and can even replace carbon from carbon dioxide and silicon from silica.
3CO2 + 4B  ----------------->  2B2O3 + 3C
3SiO2 + 4B--------------------->  2B2O3 + 3Si

5) With metals:- It combines with metals (except Cu, Ag and Au) at high temperature in the electric furnace to form borides.

6) With non-metals:- Boron combines with nitrogen, chlorine, bromine  and carbon at higher temperature forming boron nitride, BN, boron trichloride, BCl3, boron tribromide, BBr3 and boron carbide, B4C respectively. Boron carbide is probably the hardest substance known.

Borax bead test

  A pinch of borax is heated in a platinum loop, it melts to give a colourless glassy bead. It is then dipped in a coloured metallic salt solution and again heated. Characteristic coloured beads are formed. From the colour of the beads, the basic radicals are identified. Due to the formation of metallic metaborate, the characteristic colours are formed. Example: Copper salts give blue beads
In an oxidising flame
CuSO4 + B2O3 --------------->  Cu(BO2)2 + SO3

In a reducing flame
2Cu(BO2)2 + C -------------------> 2CuBO2 + B2O3 + CO
2CuBO2 + C ---------------------> 2Cu + B2O3 + CO
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