Boron 

 

Characteristics
Brown amorphous boron is a product of certain chemical reactions. It contains boron atoms randomly bonded to each other without long range order.
Crystalline boron, a very hard black material with a high melting point, exists in many polymorphs. Two rhombohedral forms, α-boron and β-boron containing 12 and 106.7 atoms in the rhombohedral unit cell respectively, and 50-atom tetragonal boron are the three most characterised crystalline forms.
Optical characteristics of crystalline/metallic boron include the transmittance of infrared light. At standard temperatures, metallic boron is a poor electrical conductor, but is a good electrical conductor at high temperatures.
Chemically boron is electron-deficient, possessing a vacant p-orbital. It is an electrophile. Compounds of boron often behave as Lewis acids, readily bonding with electron-rich substances to compensate for boron's electron deficiency. The reactions of boron are dominated by such requirement for electrons. Also, boron is the least electronegative non-metal, meaning that it is usually oxidized (loses electrons) in reactions.
Boron is also similar to carbon with its capability to form stable covalently bonded molecular networks

Physical properties

Phase

solid

Density (near r.t.)

2.34 g·cm−3

Liquid density at m.p.

2.08 g·cm−3

Melting point

2349 K
(2076 °
C, 3769 °F)

Boiling point

4200 K
(3927 °
C, 7101 °F)

Heat of fusion

50.2 kJ·mol−1

Heat of vaporization

480 kJ·mol−1

Heat capacity

(25 °C) 11.087 J·mol−1·K−1

Boron (B)

 

 

Vapor pressure

P/Pa

1

10

100

1 k

10 k

100 k

at T/K

2348

2562

2822

3141

3545

4072

Atomic properties

Crystal structure

rhombohedral

Oxidation states

3
(mildly
acidic oxide)

Electronegativity

2.04 (Pauling scale)

Ionization energies
(
more)

1st: 800.6 kJ·mol−1

2nd: 2427.1 kJ·mol−1

3rd: 3659.7 kJ·mol−1

Atomic radius

85 pm

Atomic radius (calc.)

87 pm

Covalent radius

82 pm

 

APPLICATIONS

In automobiles: it is proposed that by reacting water with the element, hydrogen could be produced to be burnt in an internal combustion engine or fed to a fuel cell to generate electricity.

10B and 11B NMR spectroscopy
Both 10B (18.8 percent) and 11B (81.2 percent) possess nuclear spin; that of boron-10 has a value of 3 and that of boron-11, 3/2. These isotopes are, therefore, of use in nuclear magnetic resonance spectroscopy; and spectrometers specially adapted to detecting the boron-11 nucleus are available commercially. The boron-10 and boron-11 nuclei also cause splitting in the resonances of attached nuclei.

B-11 depleted boron
The 10B isotope is good at capturing thermal neutrons from cosmic radiation. It then undergoes fission - producing a gamma ray, an alpha particle, and a lithium ion. When this happens inside of an integrated circuit, the fission products may then dump charge into nearby chip structures, causing data loss (bit flipping, or single event upset). In critical semiconductor designs, depleted boron—consisting almost entirely of 11B—is used to avoid this effect, as one of radiation hardening measures. 11B is a by-product of the nuclear industry.

B-10 enriched boron
The 10B isotope is good at capturing thermal neutrons, and this quality has been used in both radiation shielding and in neutron capture medical therapy where a tumor is treated with a compound containing 10B is attached to a tissue, and the patient treated with a relatively low dose of thermal neutrons which go on to cause energetic and short range alpha radiation in the tissue treated with the boron isotope.

In nuclear reactors, 10B is used for reactivity control and in emergency shutdown systems. It can serve either function in the form of borosilicate rods or as boric acid. In pressurized water reactors, boric acid is added to the reactor coolant when the plant is shut down for refueling. It is then slowly filtered out over many months as fissile material is used up and the fuel becomes less reactive.

In future manned interplanetary spacecraft, 10B has a theoretical role as structural material (as boron fibers or BN nanotube material) which also would serve a special role in the radiation shield. One of the difficulties in dealing with cosmic rays which are mostly high energy protons, is that some secondary radiation from interaction of cosmic rays and spacecraft structural materials, is high energy spallation neutrons. Such neutrons can be moderated by materials high in light elements such as structural polyethylene, but the moderated neutrons continue to be a radiation hazard unless actively absorbed in a way which dumps the absorption energy in the shielding, far away from biological systems. Among light elements that absorb thermal neutrons, 6Li and 10B appear as potential spacecraft structural materials able to do double duty in this regard.
 

 

 

 

 

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