Thallium (Tl)
Thallium (Tl) is a post-transition metal with atomic number 81 and a standard atomic weight of 204.38. At room temperature it is a solid that melts at 304 °C and boils at 1,473 °C.
Thallium key facts
Thallium at a glance
Identity
| Symbol | Tl |
|---|---|
| Atomic number (Z) | 81 |
| Atomic weight | 204.38 ± 0.01 (CIAAW 2024) |
| Category | Post-transition metal |
| Group | 13 — boron group |
| Period | 6 |
| Block | p-block |
| CAS number | 7440-28-0 |
Atomic properties
| Electron configuration | [Xe] 4f14 5d10 6s2 6p1 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p1 |
|---|---|
| Electrons per shell | 2, 8, 18, 32, 18, 3 |
| Oxidation states | +1, +3 Less common: -5, -2, -1, +2 |
| Electronegativity (Pauling) | 1.62 |
| First ionisation energy | 6.108 eV (589.4 kJ/mol) 2nd: 20.428 eV · 3rd: 29.852 eV |
| Electron affinity | 0.377 eV |
| Atomic radius (empirical) | 190 pm |
| Covalent radius | 144 pm |
| Van der Waals radius | 196 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 304 °C · 579.2 °F · 577.15 K |
| Boiling point | 1,473 °C · 2,683.4 °F · 1,746.15 K |
| Density | 11.8 g/cm³ |
| Specific heat capacity | 0.129 J/(g·K) |
| Thermal conductivity | 46.1 W/(m·K) |
| Crystal structure | Hexagonal |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 205Tl 70.485%, 203Tl 29.515% |
| Stable isotopes | 2 |
| Known isotopes | 43 |
| Abundance in Earth's crust | 0.85 mg/kg |
| Abundance in seawater | 0.000019 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Thallium electron configuration
Thallium has 81 electrons. Written in short form, its ground-state configuration is [Xe] 4f14 5d10 6s2 6p1 — the electrons of the noble gas xenon, plus 4f14 5d10 6s2 6p1.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p1.
As a member of group 13 it has 3 valence electrons in its outer shell (shell 6), which largely decide its chemistry.
Discovery and name
1861 — William Crookes · England.
Greek thallos, green shoot — for its bright green spectral line.
Known for
Highly toxic; formerly used in rat poison. Thallium-201 is used in heart scans, and thallium compounds in infrared optics.
Isotopes of thallium
Naturally occurring thallium is a mix of 2 isotopes. Each has 81 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 205Tl | 81 | 124 | 70.485% | stable |
| 203Tl | 81 | 122 | 29.515% | stable |
43 isotopes of thallium are known in our data set (half-lives as published in nuclear data tables).
Thallium compared with group 13
The other elements in group 13 (boron group), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Boron | 5 | 10.81 | 2,077 °C | 4,000 °C | 2.34 g/cm³ | 2.04 |
| Aluminum | 13 | 26.982 | 660 °C | 2,519 °C | 2.7 g/cm³ | 1.61 |
| Gallium | 31 | 69.723 | 30 °C | 2,229 °C | 5.91 g/cm³ | 1.81 |
| Indium | 49 | 114.82 | 157 °C | 2,027 °C | 7.31 g/cm³ | 1.78 |
| Thallium | 81 | 204.38 | 304 °C | 1,473 °C | 11.8 g/cm³ | 1.62 |
| Nihonium | 113 | [286] | — | — | 16 g/cm³ (pred.) | — |