Discovered in 1808 by L.J. Lussac and L.J. Thenard (in Paris) and Sir Humphrey Davy (in London).

Boron is a non-metallic element which occurs in several allotropes. It is rarely found in nature, normally occurring as borates or orthoboric acid (the abundance of boron in the earth’s crust is 10 ppm, the principal ore being borax, Na2B4O7.xH2O. Amorphous boron is the more common allotrope and exists as a dark powder which is unreactive towards water, oxygen, acids and alkalis. Boron finds importance within nuclear reactors due to its neutron absorbing capabilities, boron steel being used as control rod material.

Boron compounds are used for a number of applications including the manufacture of certain grades of glass and detergents.

Boron will react directly with most metals to produce metal borides which are hard, inert binary compounds of various formulae and arrangements of the boron atoms. For example, as single atoms (M2B), pairs, (M3B2), single and double chains (MB, M3B4), sheets (MB2), B6 octahedra (MB6) and B12 clusters (MB12).

Boron also forms the binary compound, boron nitride, which is of interest as it is isoelectronic with carbon and occurs in two structural modifications; one is a layer structure similar to graphite which is soft and lubricating, whilst the other (formed under high pressure) has a very hard, stable, tetrahedral structure as found in diamond.
Key Properties

Atomic Properties

Atomic number


5




Atomic radius – Goldschmidt


0.097nm




Atomic weight


10.81amu




Crystal structure


Tetragonal




Electronic structure


He 2s2 2 p1




Photo-electric work function


4.5eV




Thermal neutron absorption cross-section


672Barns




Valences shown


3




Neutral Isotope Distribution


Mass No.


%




10


19.8




11


80.2

Ionisation Potential


No.


eV




1


8.30




2


25.2




3


37.9




4


259




5


340

Physical Properties







Boiling point


3700°C




Density at 20°C


2.34-2.37g.cm-3




Melting point


2180°C




Electrical Properties







Electrical resistivity at 27°C


1.8x1012μOhm.cm




Thermal Properties







Latent heat of evaporation


35000J.g-1




Latent heat of fusion


2090J.g-1




Linear expansion co-efficient 0-100°C


8.3x10-6K-1




Specific heat at 25°C


1030J.K-1.kg-1




Mechanical Properties







Material condition


Arc melted




Hardness – Mohs


9.5




Tensile modulus


441GPa




Tensile strength


1580-2410MPa