81TlThallium204.38 u
Thallium (Tl)
Thallium is a post-transition metal with atomic number 81, in period 6 and group 13 of the periodic table. Its atomic mass is 204.38 u and its electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹. Solid at 25 °C.
Key facts
| Atomic number | 81 |
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| Atomic mass | 204.38 u |
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| Category | Post-Transition Metal |
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| Period | 6 |
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| Group | 13 |
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| Block | p |
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| Electron configuration | [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹ |
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| Electrons per shell | 2, 8, 18, 32, 18, 3 |
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| Electronegativity (Pauling) | 1.62 |
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| Oxidation states | −5, −2, −1, +1, +2, +3common in bold |
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| State at 25 °C | Solid |
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| Melting point | 577 K303.85 °C |
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| Boiling point | 1746 K1472.85 °C |
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| Density | 11.85 g/cm³ |
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Atomic properties
| First ionization energy | 6.108 eV |
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| Electron affinity | 19.2 kJ/mol |
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| Atomic radius | 171 pm |
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| Covalent radius | 145 pm |
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| Van der Waals radius | 196 pm |
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| Common ions | Tl⁺, Tl³⁺ |
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| Protons, neutrons, electrons | 81, 123, 81 |
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Physical and thermal properties
| Molar volume | 17.22 cm³/mol |
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| Heat of fusion | 4.2 kJ/mol |
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| Heat of vaporization | 165 kJ/mol |
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| Specific heat | 129 J/(kg·K) |
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| Thermal conductivity | 46 W/(m·K) |
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| Thermal expansion | 2.99 × 10⁻⁵ K⁻¹ |
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| Mohs hardness | 1.2 |
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| Young’s modulus | 8 GPa |
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| Crystal structure | Simple hexagonal |
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| Magnetic type | Diamagnetic |
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Identification and history
| CAS number | 7440-28-0 |
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| Latin name | Thallium |
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| Discovered by | William Crookes · 1861 |
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| Named by | --- |
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Natural isotopes
| Isotope | Mass (u) | Abundance |
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| 203Tl | 202.972344 | 29.515 % |
| 205Tl | 204.974427 | 70.485 % |
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