Thallium
POST-TRANSITION METAL · GROUP 13 · PERIOD 6
81
Tl
Thallium
204.38
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
{{BOHR_SVG}}
[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)
Thallium
0.85 ppm
Silicon (ref.)
277,000 ppm
Oxygen (ref.)
461,000 ppm
Universe (ppm by mass)
Thallium
0.1 ppm
Helium (ref.)
230,000 ppm
Hydrogen (ref.)
739,000 ppm
Discovery & History
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Safety & Handling
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Thallium in the Real World
Real-World Uses
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Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.