Thallium (Tl) – element 81
Thallium is a chemical element with the symbol Tl and atomic number 81. It is a post-transition metal in group 13, period 6 of the periodic table (p-block), and is a solid at room temperature.
Thallium (Tl) has atomic number 81, atomic mass 204.38 u and electron configuration [Xe] 4f14 5d10 6s2 6p1.
Thallium facts
| Symbol | Tl |
|---|---|
| Atomic number | 81 |
| Atomic mass | 204.38 u (g/mol) |
| Category | post-transition metal |
| Group | group 13 |
| Period | 6 |
| Block | p-block |
| Phase at 20 °C | Solid |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p1 |
| Noble-gas notation | [Xe] 4f14 5d10 6s2 6p1 |
| Electrons per shell | 2, 8, 18, 32, 18, 3 |
| Valence electrons | 3 |
Bohr model and shells
Thallium has 81 electrons in 6 shells: 2, 8, 18, 32, 18, 3. Full configuration and noble-gas notation are in the table; work out any element or ion with the electron configuration calculator.
Physical and atomic properties
| Melting point | 577 K (303.9 °C) |
|---|---|
| Boiling point | 1746 K (1472.8 °C) |
| Density | 11.85 g/cm³ |
| Electronegativity (Pauling) | 1.62 |
| Electron affinity | 36.4 kJ/mol |
| Ionization energies | 589.4, 1971, 2878 kJ/mol (1st, 2nd, 3rd) |
| Molar heat capacity | 26.32 J/(mol·K) |
| Discovered by | William Crookes |
Use the atomic mass in the molar mass calculator for any formula containing Tl.
Position in the periodic table
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Frequently asked questions
What is the atomic mass of thallium?
The atomic mass of thallium is 204.38 u, so one mole of thallium atoms weighs 204.38 g.
What is the electron configuration of thallium?
Thallium has 81 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹, or [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹ in noble-gas notation. Electrons per shell: 2, 8, 18, 32, 18, 3.
How many valence electrons does thallium have?
Thallium has 3 valence electrons (group 13).
Is thallium a metal?
Yes. Thallium is a post-transition metal.
Where is thallium on the periodic table?
Thallium is in group 13, period 6, in the p-block.
Data: Periodic-Table-JSON (CC BY-SA 3.0); standard atomic weights from IUPAC (abridged). Values in brackets are the mass number of the longest-lived isotope.