Thallium (Tl)
Post-transition metal
Thallium is a soft, grey post-transition metal with a dark reputation: its colourless, tasteless salts made it an infamous poison before regulation. Modern uses are narrow — infrared optics and low-temperature superconductor research.
Position on the periodic table
Atomic properties
| Atomic number | 81 |
|---|---|
| Atomic mass | 204.38 amu |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p1 |
| Noble gas shorthand | [Xe] 4f14 5d10 6s2 6p1 |
| Electrons per shell | 2, 8, 18, 32, 18, 3 |
| Valence electrons (outer shell) | 3 |
| Common oxidation states | +1, +3 |
| Electronegativity (Pauling) | 1.62 |
| Covalent radius (approx.) | 145 pm |
| First ionization energy | 589.4 kJ/mol |
| Electron affinity | 36.4 kJ/mol |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Density | 11.85 g/cm³ |
| Melting point | 303.85 °C (577 K) |
| Boiling point | 1472.8 °C (1746 K) |
| Appearance | silvery white |
| Radioactive | No |
| Origin | Occurs naturally |
Electron configuration of thallium
Thallium's ground-state electron configuration is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p1, usually shortened to [Xe] 4f14 5d10 6s2 6p1. Its electrons occupy 6 shells (2, 8, 18, 32, 18, 3), placing it in period 6 of the p-block. The 3 outer-shell electrons drive its bonding behaviour. Explore it interactively in the electron configuration calculator.
Uses of thallium
- Infrared lenses and detectors
- Low-melting specialty glass
- Medical cardiac imaging (Tl-201, declining)
Biological role: None — highly toxic; it mimics potassium in the body.
Occurrence: Trace by-product of zinc and lead smelting.
Common compounds of thallium
Tl2SO4
Thallium(I) sulfate
History and discovery
Discovered: 1861 — William Crookes. Name origin: From Greek thallos, “green shoot”, for its bright green spectral line.
William Crookes spotted thallium's green spectral line in 1861; Claude-Auguste Lamy isolated the metal a year later.
Safety notes
Extremely toxic — historic thallium rat poisons are banned in most countries.
Educational context only — always follow your school's laboratory rules and never handle chemicals without proper supervision. See our disclaimer.
Practice questions
Quick practice: Thallium
1. What is the chemical symbol of Thallium?
2. What is the atomic number of Thallium?
3. Which category does Thallium belong to?
4. What is the state of Thallium at room temperature?
5. Which period of the periodic table is Thallium in?
Thallium FAQs
What is the atomic number of thallium?
Thallium's atomic number is 81 — every thallium atom has 81 protons in its nucleus.
What is the symbol for thallium?
Tl. From Greek thallos, “green shoot”, for its bright green spectral line.
Is thallium a metal, nonmetal, or metalloid?
Thallium is classified as a post-transition metal.
What state is thallium at room temperature?
At about 25 °C, thallium is a solid. It melts at 303.85 °C.
How many valence electrons does thallium have?
Thallium has 3 electrons in its outer shell (shell pattern: 2, 8, 18, 32, 18, 3).
What is thallium used for?
Key uses include: infrared lenses and detectors; low-melting specialty glass; medical cardiac imaging (tl-201, declining).
Related elements
Keep working with this element
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