Boron (B)

Stable Isotopes of Boron

Isotope Atomic Mass Natural Abundance Nuclear Spin Nuclear Magnetic Moment
B-10 10.0129370 19.90% 3 1.80065
B-11 11.0093055 80.10% 3/2 2.688637



Boron was discovered in 1808 by Sir Humphry Davy, Joseph Louis Gay-Lussac and Louis Jacques Thénard. Its name derives from the Arabic word buraq or the Persian word burah, both of which are names for the mineral borax.

Boron is a nonmetallic element, either a black, hard solid or a brown, amorphous powder. It is one of the least reactive elements on the periodic table; it does not react with water at ambient temperatures, although the powdered amorphous form reacts slowly at 100 ºC, producing boric acid. It reacts vigorously with concentrated nitric acid and ignites in oxygen at 700 ºC. 

The amorphous boron metal reacts slowly with dilute mineral acids at ambient temperatures. Boron also reacts with halogens to form boron halides, instantaneously with fluorine but at elevated temperatures with other halogens (chlorine, bromine and iodine, at 400 ºC, 600 ºC, and 900 ºC respectively). 

Enriched boron is used in radiation shielding and is the primary nuclide used in neutron capture therapy of cancer. Boron-10 is used in nuclear reactors for reactivity control and in emergency shutdown systems. Boric acid is added to reactor coolant when the plant is shut down for refueling. Boron-10 and -11 are of use in nuclear magnetic resonance (NMR) spectroscopy.

In its elemental form, boron is nontoxic — it is actually a plant micronutrient. Rubbing the amorphous powder on the skin, however, can produce irritation. Some compounds of boron are poisonous. 

Properties of Boron

Name Boron
Symbol B
Atomic number 5
Atomic weight 10.811
Standard state Solid at 298 °K
CAS Registry ID 7440-42-8
Group in periodic table 13 
Group name None 
Period in periodic table
Block in periodic table p-block 
Color Black 
Classification Semi-metallic 
Melting point 2075 °C 
Boiling point 2550 °C 
Vaporization point 4000 °C
Thermal conductivity 0.274  W/(m·K) at  298.2 °K
Electrical resistivity 1.8 x 1012 µΩ·cm at 0 ºC; 3.0 x 106 Ω·cm at 100 ºC 
Electronegativity 2.0 
Specific heat 1030 J/(kg·K)
Heat of vaporization 507 kJ·mol-1 at 2550 ºC 
Heat of fusion 50 kJ·mol-1 
Density of solid 2.34 cryst. g/cm3
Density of powder 2.45 g/cm3
Electron configuration [He]2s22p
Oxidation state  +3 
Mohs hardness scale 9.3 

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