Thallium
Thallium is a chemical element with the symbol Tl and atomic number 81. It has a standard atomic weight of 204.38, sits in period 6 and group 13, and is classified as a post-transition metal.
Properties
| Symbol | Tl |
|---|---|
| Atomic number | 81 |
| Standard atomic weight | 204.38 |
| Category | Post-transition metal |
| Group | 13 |
| Period | 6 |
| Electron configuration | [Xe] 4f14 5d10 6s2 6p1 |
| Outer-shell electrons | 3 |
The figure of 204.38 is a conventional value, because IUPAC publishes an interval for this element rather than a single weight. For elements like this one the fourth digit carries real uncertainty, because the isotopic composition varies measurably between natural sources — so quoting eight digits would overstate what is known.
Calculate with thallium
- Molar mass of any formula containing it
- Electron configuration, including ions
- Effective nuclear charge by Slater's rules
- Average atomic mass from isotope abundances
Group 13
Atomic weights are the IUPAC 2021 standard values, abridged to the conventional five significant figures. Group, period, category and electron configuration are derived from the atomic number rather than stored, so they cannot drift out of step with it.