Boron

5

B

10.811(7)
  • Name: Boron
  • Symbol: B
  • Atomic number: 5
  • Atomic weight: 10.811(7)
  • CAS Registry ID: 7440-42-8
  • Group name: (none)
  • Period number: 2
  • Block: p-block
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Description

Boron is a semimetal which has an atomic weight of 10.8111. Boron has a melting point of 3760 °F and a boiling point of 7101 °F. In its natural state, boron is hard and brittle and has a shiny black appearance; at high temperatures boron is an excellent conductor of electricity. Uses of boron include: in the natural form of a borate named borax for cleaning solutions and as a natural remedy for insects infestations, in the form of amorphous boron to give the green color in firework displays, and to provide shields against nuclear radiation.

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Isolation

Pure boron is not found free in nature and it must be isolated through chemical processes. The main sources of boron are through borax deposits that are found in Turkey, and the Mojave Desert in California.

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Boron is available commercially and so it is not isolated through regular laboratory procedures. There are different procedures that can be used to isolate boron for commercial uses on a large scale. Large amounts of impure boron are isolated by reducing boric anhydride with magnesium through a thermite reaction. Pure crystalline boron is isolated through reducing boron trichloride and hydrogen through vapor phase reduction using hydrogen on an electrically heated filament flow system. Other isolation procedures yield small amounts of pure boron by using the thermal decomposition of the chemical BBr3 ; the chemical boron compound is heated with hydrogen over a tantalum wire to temperatures in excess of 1832 °F. Another isolation procedure involves creating boric acid from borax sources to form a boron oxide; the boric acid is obtained from the borax sources and melted to create boric oxide. The oxide is then reacted with three parts of magnesium to yield pure boron and magnesium oxide.
General
Name Boron
Symbol B
Number 5
Chemical series metalloids
Group 13
Period 2
Block p
Appearance black/brown
Standard atomic weight 10.811(7) g·mol−1
Electron configuration 1s2 2s2 2p1
Electrons per shell 2, 3
Atomic properties
Crystal structure rhombohedral
Oxidation states 3 (mildly acidic oxide)
Electronegativity 2.04 (Pauling scale)
Ionization energies 1st: 800.6 kJ·mol−1
(more) 2nd: 2427.1 kJ·mol−1
(more) 3rd: 3659.7 kJ·mol−1
Atomic radius 85 pm
Atomic radius (calc.) 87 pm
Covalent radius 82 pm
Van der Waals radius
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)
Critical point
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
Miscellaneous
Magnetic ordering nonmagnetic
Electrical resistivity (20 °C) 1.5×104Ω·m
Thermal conductivity (300 K) 27.4 W·m−1·K−1
Thermal expansion (25 °C) 5–7 µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 16200 m/s
Young's modulus
Shear modulus
Bulk modulus (β form) 185 GPa
Poisson ratio
Mohs hardness 9.3
Vickers hardness 49000 MPa
Brinell hardness
CAS registry number 7440-42-8

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