5BBoron10.81 u
Boron (B)
Boron is a metalloid with atomic number 5, in period 2 and group 13 of the periodic table. Its atomic mass is 10.81 u and its electron configuration is [He] 2s² 2p¹. Solid at 25 °C.
Key facts
| Atomic number | 5 |
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| Atomic mass | 10.81 u |
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| Category | Metalloid |
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| Period | 2 |
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| Group | 13 |
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| Block | p |
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| Electron configuration | [He] 2s² 2p¹ |
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| Electrons per shell | 2, 3 |
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| Electronegativity (Pauling) | 2.04 |
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| Oxidation states | −5, −3, −1, +1, +2, +3common in bold |
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| State at 25 °C | Solid |
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| Melting point | 2348 K2074.85 °C |
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| Boiling point | 4273 K3999.85 °C |
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| Density | 2.37 g/cm³ |
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Atomic properties
| First ionization energy | 8.298 eV |
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| Electron affinity | 26.7 kJ/mol |
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| Atomic radius | 87 pm |
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| Covalent radius | 84 pm |
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| Van der Waals radius | 192 pm |
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| Common ions | B³⁺ |
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| Protons, neutrons, electrons | 5, 6, 5 |
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Physical and thermal properties
| Molar volume | 4.39 cm³/mol |
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| Heat of fusion | 50 kJ/mol |
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| Heat of vaporization | 507 kJ/mol |
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| Specific heat | 1030 J/(kg·K) |
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| Thermal conductivity | 27 W/(m·K) |
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| Thermal expansion | 6 × 10⁻⁶ K⁻¹ |
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| Mohs hardness | 9.3 |
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| Young’s modulus | --- |
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| Crystal structure | Rhombohedral |
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| Magnetic type | Diamagnetic |
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Identification and history
| CAS number | 7440-42-8 |
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| Latin name | Borum |
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| Discovered by | J. Gay-Lussac, L. Jacques · 1808 |
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| Named by | --- |
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Natural isotopes
| Isotope | Mass (u) | Abundance |
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| 10B | 10.012937 | 19.65 % |
| 11B | 11.009305 | 80.35 % |
Data checked against PubChem (NIH), IUPAC, the NIST Atomic Spectra Database and the IAEA (NUBASE and AME), the same data you see in the app.
A dash (---) means there is no reliable published measurement. The app never makes one up.
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Periodic Table