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Boron,  ₅B

General properties
Allotropes	α-, β-rhombohedral, β-tetragonal (and more)
Appearance	black-brown
Boron in the periodic table
– ↑ B ↓ Al beryllium ← boron → carbon ‍
Atomic number (Z)	5
Group, period	group 13 (boron group), period 2
Block	p-block
Electron configuration	{{{electron configuration}}}
Electrons per shell	2, 3
Physical properties
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 |- ! style="text-align:right;" | 10B | style="text-align:right" | 20% | colspan="3" style="text-align:left;" | is stable |- ! style="text-align:right;" | 11B | style="text-align:right" | 80% | colspan="3" style="text-align:left;" | is stable P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 2348 2562 2822 3141 3545 4072
Atomic properties
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
Miscellanea
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 (χmol)	−6.7·10−6 cm3/mol
Mohs hardness	~9.5
CAS Number	7440-42-8
تاریخی
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 Decay mode Pro­duct
10B content is 19.1–20.3% in natural samples, with the remainder being 11B.

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