81204.38TlThallium
Thallium (Tl) — element 81post-transition metal · group 13, period 6
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 (the p-block), with a standard atomic mass of 204.38 u. At room temperature (25 °C) thallium is a solid.
💡 Known for: A highly toxic metal once used in rat poison; also used in some infrared optics.
# Thallium properties
| Symbol | Tl |
|---|---|
| Atomic number | 81 |
| Atomic mass | 204.38 u |
| Category | post-transition metal |
| Group | 13 |
| Period | 6 |
| Block | p-block |
| Electron configuration | [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹ |
| Full configuration | 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p¹ |
| Electrons per shell | 2, 8, 18, 32, 18, 3 |
| Protons / electrons | 81 / 81 |
| State at 25 °C | solid |
| Melting point | 577 K (303.9 °C, 578.9 °F) |
| Boiling point | 1746 K (1472.8 °C, 2683.1 °F) |
| Density | 11.85 g/cm³ |
| Electronegativity (Pauling) | 1.62 |
| First ionization energy | 6.108 eV |
| Atomic radius | 156 pm |
| Discovered | 1861 |
# Electron configuration of thallium
The electron configuration of thallium is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹, written out in full as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p¹. Its 81 electrons fill 6 shells as 2, 8, 18, 32, 18, 3, so the outermost shell holds 3 electrons. As a group 13 element, thallium has 3 valence electrons.
# Related elements
HgMercuryprevious elementPbLeadnext elementBBorongroup 13AlAluminiumgroup 13GaGalliumgroup 13InIndiumgroup 13NhNihoniumgroup 13SnTinpost-transition metalBiBismuthpost-transition metalPoPoloniumpost-transition metalFlFleroviumpost-transition metalMcMoscoviumpost-transition metalLvLivermoriumpost-transition metal
# Thallium FAQ
What is the atomic number of thallium?
The atomic number of thallium is 81: every thallium atom has 81 protons in its nucleus. Its symbol is Tl.
What is the atomic mass of thallium?
The standard atomic mass of thallium is 204.38 u.
What is the electron configuration of thallium?
Thallium's electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹ (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p¹), with 2, 8, 18, 32, 18, 3 electrons per shell.
Is thallium a metal?
Yes. Thallium is a post-transition metal, one of the metals.
What is the melting point of thallium?
Thallium melts at 577 K (303.9 °C, 578.9 °F) and boils at 1746 K (1472.8 °C, 2683.1 °F).
What is thallium used for / known for?
A highly toxic metal once used in rat poison; also used in some infrared optics.