Thallium (Tl)
Element 81 · Post-transition metal · Solid at 25 °C · Group 13, Period 6
Thallium is a soft, highly toxic metal; its modern uses are narrow and specialized, in optics and electronics research.
On the periodic table, thallium sits in group 13 — the boron group — in period 6. It is one of 12 elements classified as a post-transition metal, and at room temperature (25 °C) it is a solid.
The post-transition metals sit between the transition metals and the metalloids. They are genuine metals — shiny and conductive — but softer, weaker, and lower-melting than the d-block metals, and their chemistry has more covalent character. Everyday examples from this family include aluminium foil, tin cans, and lead batteries.
Properties of thallium
| Atomic number (Z) | 81 |
|---|---|
| Symbol | Tl |
| Atomic mass | 204.38 |
| Category | Post-transition metal |
| Group | 13 |
| Period | 6 |
| Block | p-block |
| Metal / nonmetal | Metal |
| State at 25 °C | Solid |
| Electron configuration | [Xe] 4f14 5d10 6s2 6p1 |
| Electronegativity (Pauling) | 1.62 |
| Melting point | 304 °C (577.15 K / 579.2 °F) |
| Boiling point | 1,473 °C (1,746.15 K / 2,683.4 °F) |
| Density | 11.85 g/cm³ |
Position in the periodic table
Thallium shares group 13 — the boron group with Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), Nihonium (Nh). Elements in the same group have the same number of outer electrons, so they tend to behave alike chemically — comparing them is one of the fastest ways to see periodic trends in action.
Physical properties
Thallium melts at 304 °C (577.15 K / 579.2 °F) and boils at 1,473 °C (1,746.15 K / 2,683.4 °F), giving it a liquid range of about 1,169 °C at standard pressure.
Its density is 11.85 g/cm³.
See also: the interactive periodic table, how to read the periodic table, metals, nonmetals & metalloids.