Thallium (Tl)
Thallium is element 81, a post-transition metal in period 6, group 13 of the periodic table. At room temperature it is a solid.
Electron configuration [Xe] 4f14 5d10 6s2 6p1
Electrons per shell 2, 8, 18, 32, 18, 3
Thallium data
| Atomic number | 81 |
|---|---|
| Symbol | Tl |
| Atomic mass | 204.383 u |
| Category | Post-transition metal |
| Group | 13 |
| Period | 6 |
| Block | p |
| State at 25 °C | solid |
| Melting point | 577 K (303.9 °C) |
| Boiling point | 1746 K (1472.9 °C) |
| Density | 11.85 g/cm³ |
| Electronegativity (Pauling) | 2.04 |
| Atomic radius | 148 pm |
| 1st ionization energy | 589 kJ/mol |
| Electron affinity | -19 kJ/mol |
| Oxidation states | +1, +3 |
| Common ion charge | +, 3+ |
| Discovered | 1861 |
Orbital diagram
Each box is one orbital holding up to two electrons of opposite spin. Following Hund’s rule, electrons spread out singly across a subshell before pairing.
Work through thallium’s configuration and its ions →
Where thallium sits
Thallium belongs to the post-transition metals. Its Pauling electronegativity of 2.04 is mid-range; see the electronegativity chart and periodic trends for context. To use its mass in a calculation, try the molar mass calculator.
Thallium FAQ
What is the electron configuration of thallium?
Thallium (Z = 81) has the configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 4f14 5d10 6s2 6p1. In noble-gas shorthand that is [Xe] 4f14 5d10 6s2 6p1.
How many valence electrons does thallium have?
3 (outer shell (n = 6)).
What is the atomic mass of thallium?
The standard atomic weight of thallium is 204.383 u, so one mole weighs 204.383 g.
Is thallium a metal?
Yes. Thallium is a post-transition metal.
What is the charge of a thallium ion?
Thallium most commonly forms Tl+ or Tl3+.