55CsCesium132.90545196 u
Cesium (Cs)
Cesium is an alkali metal with atomic number 55, in period 6 and group 1 of the periodic table. Its atomic mass is 132.90545196 u and its electron configuration is [Xe] 6s¹. Solid at 25 °C.
Key facts
| Atomic number | 55 |
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| Atomic mass | 132.90545196 u |
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| Category | Alkali Metal |
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| Period | 6 |
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| Group | 1 |
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| Block | s |
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| Electron configuration | [Xe] 6s¹ |
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| Electrons per shell | 2, 8, 18, 18, 8, 1 |
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| Electronegativity (Pauling) | 0.79 |
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| Oxidation states | −1, +1common in bold |
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| State at 25 °C | Solid |
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| Melting point | 301.59 K28.44 °C |
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| Boiling point | 944 K670.85 °C |
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| Density | 1.93 g/cm³ |
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Atomic properties
| First ionization energy | 3.894 eV |
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| Electron affinity | 45.5 kJ/mol |
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| Atomic radius | 267 pm |
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| Covalent radius | 244 pm |
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| Van der Waals radius | 343 pm |
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| Common ions | Cs⁺ |
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| Protons, neutrons, electrons | 55, 78, 55 |
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Physical and thermal properties
| Molar volume | 70.94 cm³/mol |
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| Heat of fusion | 2.09 kJ/mol |
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| Heat of vaporization | 64 kJ/mol |
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| Specific heat | 242 J/(kg·K) |
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| Thermal conductivity | 36 W/(m·K) |
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| Thermal expansion | 9.26 × 10⁻⁵ K⁻¹ |
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| Mohs hardness | 0.2 |
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| Young’s modulus | 1.7 GPa |
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| Crystal structure | Body-centered cubic |
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| Magnetic type | Paramagnetic |
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Identification and history
| CAS number | 7440-46-2 |
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| Latin name | Cesium |
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| Discovered by | R. Bunsen, G. Kirchhoff · 1860 |
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| Named by | --- |
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Natural isotopes
| Isotope | Mass (u) | Abundance |
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| 133Cs | 132.905452 | 100 % |
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