Boron has atomic number 5. So, the electronic configuration would be 1s2 2s2 2p1. It has 3 electrons in the outer most shell. So, it has a valency of 3.
Boron exists in two allotropic forms. These are crystalline or adamantine boron, and amorphous boron. The crystalline form can be obtained by the reduction of B2O3 or KBF4 by aluminum.
B2O3 + 2Al → Al2O3 + 2B ( crystalline boron)
KBF4 + Al → AlF3 + KF + B( crystalline boron)
Crystalline boron can also be prepared by dropping amorphous boron in molten Al at 1500 C when it appears to dissolve. On cooling, yellow color crystals are deposited on the surface of Al. The mass is broken up and heated with HCl to dissolve away the Al and very hard crystals are left behind.
Boron obtained by this method is not pure since it conatins some amount of Al as an impurity. Crystaline boron is black and possesses metallic lusture. It is extremely inter. It is scarcely attacked by conc. HNO3 or H2SO4 and not at all by boiling conc HCl or HF.
Crystalline boron is very hard and is next to diamond in hardness.
Action of air and O2
Amorphous Boron is very reactive. When it is heated at 700 C in the air or O2, it burns with a reddish flame forming a mixture of oxide and nitrate.
4B + 3O2 → 2B2O3
2B + N2 → 2BN
Action of acids
Halogen acids have no action on boron but it is attacked by oxidizing acids like HNO3 and H2SO4.
B + 3HNO3 → H3BO3 + 3NO2
2B + 3H2SO4 → 2H3BO3 + 3SO2
As reducing agents
Boron is a powerful reducing agent, since it can replace carbon from CO2
, silicon from SiO2
, and nitrogen from NO.
4B + 3CO2 → 2B2O3 + 3C
4B + 3SiO2 → 2B2O3 + 3Si
4B + 6NO → 2B2O3 + 3N2
Action of Alkalies
It is vigorously attacked by fused alkalies or by fusion with oxidizing fusion mixture.
2B + 6NaOH → 2Na3BO3 + 3H2
Action of metals
Except for Cu, Ag and Au, Boron combines with other metals at high temperatures to form borides. These borides are extremely hard substances. Mg3B2 and Be3B2 are attacked by acids to form boron hydrides.
Action of non-metals
Boron also reacts with non-metals. For example: Boron combines with N2 at temperatures greater than 900 C to form boron nitride, BN. Boron forms boron sulphide B2S3 when heated with S. On heating with Carbon in an electric furnace an extremely hard substance namely boron carbide, B4C is formed.
Action of steam
Boron reacts with steam at red heat liberating H2.
2B + 3H2O → B2O3 + 3H2
Boron forms various compounds with different elements.
The main compounds of boron are,
- Boric acid
- Boron nitride
- Boron trihalides
- Ffluoroboric acid
Boron forms several boric acids. Boric acids like orthoboric acid, metaboric acid, pyroboric acid, and tetraboric acid are known. Out of these acids, orthoboric acid which is generally referred to as boric acid is the most important and is stable both in the solid state and as solution. The remaining acids are stable in the solid state and gradually change into orthoboric acid in solution. The formula of orthoboric acid or boric acid is H3BO3.
HBO2 + H2O → H3BO3
H2B4O7 + 5H2O → 4H3BO3
Small quantities of boric acid are present in the jets of steam called saffioni which is mixed with other gases in the volcanic district of Tuscany. Besides this it is also found in many mineral waters. It is also present in traces of hops, in batteries and fruits and often in wines.
- The formula of borax is Na2B4O7.10H2O. It is the sodium salt of tetraboric acid, H2B4O7.
- It occurs naturally as tincal or suhaga in the dried up lakes of Tibet, Ceylon, California and India. In India, it is found in Ladakh and the Puga valley of Kashmir.
- Tincal contains about 55% Borax. Borax can be obtained from colemanite and by the action of Na2CO3 on H3BO3.
- It is sparingly soluble in cold water and fairly soluble in hot water. The solution is alkaline in nature.
- Borazine is also known as borazole.
- It has the formula B3N3H6. This compound is isoelectronic with benzene and hence has been called inorganic benzene by Weberg. Borazine is a colorless, mobile volatile liquid. It freezes at -58 C.
- It's boiling points and melting points are: 63 C and -58 C respectively. Borazine gets slowly hydrolyzed by water to produce H2, boric acid and NH3.
- Hydrolysis is favored by increase in temperature.
- The formula is BN. It is a white powder with a density of 2.34. It melts under pressure at 3000 C.
It is a very stable and unreactive substance. It remains unaffected by mineral acids, solutions of alkalies and Cl2 at red heat.
- It gets decomposed when heated in steam to give NH3.
- It also decomposes when heated with fused KOH.
BN + 3H2O → H2BO3 + NH3
BN + KOH → K3BO3 + NH3
BN possesses the same hardness as a diamond and can withstand temperatures of more than 300 C. Due to this property it is used for coating crucible linings.
Boron reacts with halogens to form trihalides BX3, where X= F, Cl, Br, I. All the trihalides of boron except BF3 can be prepared by the treatment of a mixture of B2O3 and carbon with the appropriate halogen at higher temperature.
B2O3 + 3C + 3X2 → 2BX3 + 3CO
BF3 and BCl3 are gases, BBr3 is a liquid and BI3 is a solid at room temperature. Boron trihalides are covalent in nature due to small size and the high charge density on B3+ ion. Some other points which favor the covalent character of boron trihalides are: as liquids they do not conduct electricity, their boiling points are very low as compared to the halides of the elements of groups I A and II A, they exist as discrete molecular species.
- The formula is HBF4. It is a strong acid, stronger than HF. It is decomposed when heated.
- The salts given by this acid are fluoroborides or fluoroborates.
- These salts are prepared by dissolving the corresponding metal borides, hydroxides or carbonates in aqueous hydrofluoric acid.
- Fluoroborates are also obtained in solution by treating alkali acid fluorides with H3BO3.