Thallium (symbol Tl, atomic number 81) is a soft, highly toxic metal once used in rat poison and now in electronics. It is classified as a post-transition metal and sits in group 13, period 6 of the periodic table.
Overview
Thallium is the chemical element with atomic number 81 and the symbol Tl. With a standard atomic weight of 204.38, it is a post-transition metal. In the periodic table it occupies group 13 and period 6, and under standard conditions it is normally a solid. Thallium is a soft, highly toxic metal once used in rat poison and now in electronics.
Atomic structure
An atom of Thallium has 81 protons in its nucleus and, when neutral, 81 electrons arranged in the configuration [Xe] 4f14 5d10 6s2 6p1. Its standard atomic weight of 204.38 reflects the natural mix of its stable isotopes.
Discovery and place in the table
Thallium was discovered in 1861 by William Crookes. It follows Mercury (Hg) and precedes Lead (Pb) in order of atomic number.
| Atomic number | 81 |
|---|---|
| Symbol | Tl |
| Atomic mass | 204.38 |
| Category | Post-transition metal |
| Group | 13 |
| Period | 6 |
| Electron configuration | [Xe] 4f14 5d10 6s2 6p1 |
| Phase at STP | Solid |
| Discovered | 1861 (William Crookes) |