Thallium
Atomic Data
| Atomic Number | 81 |
| Symbol | Tl |
| Atomic Weight | 204.38 u |
| Density (STP) | 11.85 g/cm³ |
| Melting Point | 303.85 °C (577 K) |
| Boiling Point | 1472.85 °C (1746 K) |
| Electronegativity | 1.62 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p1 |
| Oxidation States | +1, +3 |
| Phase at STP | Solid |
| Category | Post-Transition Metal |
| Period / Group | 6 / 13 |
| CAS Number | 7440-28-0 |
Electron Configuration
[Xe] 4f14 5d10 6s2 6p1
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 14 | 60 |
| O | 5 | 5s | 2 | 62 |
| O | 5 | 5p | 6 | 68 |
| O | 5 | 5d | 10 | 78 |
| P | 6 | 6s | 2 | 80 |
| P | 6 | 6p | 1 | 81 |
| Total | 81 | 81 | ||
Isotopes of Thallium
Thallium has two naturally occurring stable isotopes. The most abundant is ²⁰⁵Tl, comprising 70.476% of all naturally occurring Thallium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Thallium-203 | ²⁰³Tl | 81 | 122 | 29.524 | Stable |
| Thallium-205 | ²⁰⁵Tl | 81 | 124 | 70.476 | Stable |
Abundance & Occurrence
Thallium is present in Earth's crust at approximately 0.85 ppm by mass and at approximately 0.1 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- High acute toxicity: Soluble thallium salts (thallium sulfate, thallium acetate) are among the most toxic of all common metal salts: comparable to lead or arsenic; thallium was historically used as rat poison and has been used as a murder weapon.
- Skin absorption: Thallium salts are readily absorbed through intact skin; dermal contact must be prevented with appropriate gloves: neoprene or nitrile gloves are recommended as thallium can penetrate some materials.
- Symptoms of thallium poisoning: Alopecia (hair loss) 2–3 weeks after exposure is a characteristic sign; other effects include peripheral neuropathy, encephalopathy, and visual impairment.
- Thallium-201: medical use: Tl-201 (t½ = 73 h, gamma) is used in cardiac SPECT imaging; radiopharmacy staff must follow standard radiation protection procedures; patient handling precautions are also required.
- Storage and disposal: Thallium compounds must be stored in clearly labelled, secured containers; disposal requires licensed hazardous waste management due to high toxicity.
Thallium in the Real World
Real-World Uses
- Nuclear medicine (cardiac imaging): Thallium-201 (t½ = 73.1 h) is injected intravenously during myocardial perfusion imaging; the isotope concentrates in heart muscle proportional to blood flow, and cold spots on the scan indicate coronary artery disease.
- Infrared detectors: Thallium bromoiodide (KRS-5) is a dense, infrared-transmitting crystal used as windows and prisms in infrared spectrometers and thermal imaging systems, transmitting from 0.5 to 40 μm.
- High-refractive-index glass: Thallium oxide is added to specialty optical glasses to produce exceptionally high refractive indices, used in objective lenses for microscopes and fibre-optic sensors where maximum light-bending power is needed.
- Superconductor research: Thallium barium calcium copper oxide (TBCCO) superconductors achieve critical temperatures above 125 K, among the highest for cuprate superconductors, of interest for high-field magnet and power transmission applications.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is thallium used for?
Thallium has very limited uses today due to its extreme toxicity. Thallium sulphate was once used as a rodenticide and ant killer but has been banned in most countries. Thallium-201 is used in nuclear medicine stress tests of the heart. Thallium compounds are used in speciality glasses with high refractive indices for infrared optics, and in some high-temperature superconductors.
Is thallium the most toxic element?
Thallium is among the most acutely toxic of all metals. Thallium(I) ions are toxic because they mimic potassium ions and are taken up by cells throughout the body. They interfere with potassium-dependent enzymes, particularly those affecting nerve function. Thallium poisoning causes hair loss (a characteristic sign used in murder investigations), nerve damage, and organ failure. A lethal dose can be as small as 10–15 mg/kg, making it more toxic than lead or mercury. Its notoriety as a 'poison of choice' in murder cases earned it the name 'poisoner's poison'.
How was thallium discovered?
Thallium was discovered in 1861 by British chemist William Crookes using flame spectroscopy, detecting a vivid green spectral line in the residue from sulfuric acid production. He named it thallium from the Greek 'thallos', meaning green shoot or twig, for that bright green spectral line. French chemist Claude-Auguste Lamy independently discovered it about the same time and first isolated the metal.
How is thallium-201 used in heart imaging?
Thallium-201 behaves like potassium in the body and is actively taken up by living heart muscle cells. During a nuclear stress test, a small amount of Tl-201 is injected and a gamma camera images where it concentrates. Healthy heart muscle absorbs thallium normally, while areas with poor blood flow (from coronary artery disease) show reduced uptake ('cold spots'). This allows cardiologists to identify which parts of the heart are at risk. Technetium-99m agents have largely replaced thallium-201 due to better imaging properties.