Boron,  ₅B

boron (β-rhombohedral)
General properties
Pronunciation	/ˈbɔːrɒn/ ​(BOHR-on)
Allotropes	α-, β-rhombohedral, β-tetragonal (and more)
Appearance	black-brown
Standard atomic weight (Ar, standard)	[10.806, 10.821]
Boron in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson – ↑ B ↓ Al beryllium ← boron → carbon
Atomic number (Z)	5
Group	group 13 (boron group)
Period	period 2
Block	p-block
Element category	metalloid
Electron configuration	[He] 2s2 2p1
Electrons per shell	2, 3
Physical properties
Phase at STP	B: Solid
Melting point	2349 K ​(2076 °C, ​3769 °F)
Boiling point	4200 K ​(3927 °C, ​7101 °F)
Density when liquid (at m.p.)	2.08 g/cm3
Heat of fusion	50.2 kJ/mol
Heat of vaporization	508 kJ/mol
Molar heat capacity	11.087 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 2348 2562 2822 3141 3545 4072
Atomic properties
Oxidation states	−5, −1, +1, +2, +3 (a mildly acidic oxide)
Electronegativity	Pauling scale: 2.04
Ionization energies	1st: 800.6 kJ/mol 2nd: 2427.1 kJ/mol 3rd: 3659.7 kJ/mol (more)
Atomic radius	empirical: 90 pm
Covalent radius	84±3 pm
Van der Waals radius	192 pm
Spectral lines of boron
Other properties
Natural occurrence	B: Primordial
Crystal structure	​rhombohedral
Speed of sound thin rod	16,200 m/s (at 20 °C)
Thermal expansion	β form: 5–7 µm/(m·K) (at 25 °C)
Thermal conductivity	27.4 W/(m·K)
Electrical resistivity	~106 Ω·m (at 20 °C)
Magnetic ordering	diamagnetic
Magnetic susceptibility	−6.7·10−6 cm3/mol
Mohs hardness	~9.5
CAS Number	7440-42-8
History
Discovery	Joseph Louis Gay-Lussac and Louis Jacques Thénard (30 June 1808)
First isolation	Humphry Davy (9 July 1808)
Main isotopes of boron
Iso­tope Abun­dance Half-life (1/2) Decay mode Pro­duct 10B 20% stable 11B 80% stable
10B content may be as low as 19.1% and as high as 20.3% in natural samples. 11B is the remainder in such cases.
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Boron in a tube

Boron is a chemical element. It has the chemical symbol B. It has the atomic number 5. It is a metalloid (it has properties of a metal and a non-metal). Much boron is found in chemical compounds in its ore borax. Boron is never found free in nature.