Element 81 · Post-transition metal
Thallium (Tl) — Element 81
Greek thallos — green shoot
Thallium (Tl) is a post-transition metal in Period 6, Group 13 of the periodic table with atomic number 81, standard atomic weight 204.38 u, and ground-state electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹. At standard temperature and pressure it is classified as a solid with 3 valence electrons and common oxidation states +1, +3.
Electronics, optics, research
What are the atomic properties and physical constants of Thallium?
Reference constants compiled from IUPAC standard atomic weights, the NIST Atomic Spectra Database, and the CRC Handbook.
- Atomic number (Z)
- 81
- Standard atomic mass
- 204.38 u
- Chemical family
- Post-transition metal
- Ground-state configuration
- [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹
- Electrons per shell
- 2, 8, 18, 32, 18, 3
- Valence electrons
- 3
- Common ion charges / oxidation states
- +1, +3
- Phase at STP
- Solid
- Pauling electronegativity
- 1.62 Pauling
- First ionization energy
- 589 kJ/mol
- Atomic radius
- 156 pm
- Melting point
- 304 °C
- Boiling point
- 1,473 °C
- Discovery year
- 1861
- Table coordinate
- Period 6, Group 13
- Radioactivity
- Has stable isotope(s)
How are electrons arranged across Thallium’s shells?
A neutral Thallium atom contains 81 electrons distributed across 6 occupied principal energy shells ([Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹). The table below compares Thallium’s shell occupancy against each shell’s theoretical 2n² capacity.
| Principal Shell (n) | Spectroscopic Letter | Electrons in Tl | Max Shell Capacity (2n²) |
|---|---|---|---|
| n = 1 | K | 2 e⁻ | 2 e⁻ |
| n = 2 | L | 8 e⁻ | 8 e⁻ |
| n = 3 | M | 18 e⁻ | 18 e⁻ |
| n = 4 | N | 32 e⁻ | 32 e⁻ |
| n = 5 | O | 18 e⁻ | 50 e⁻ |
| n = 6 | P | 3 e⁻ | 72 e⁻ |
Frequently Asked Questions about Thallium (Tl)
What is the atomic mass and atomic number of Thallium (Tl)?
Thallium (Tl) has atomic number Z = 81 (81 protons in its nucleus) and an IUPAC standard atomic weight of 204.38 u. It sits in Period 6, Group 13 and is classified as a post-transition metal.
What is the ground-state electron configuration of Thallium?
The neutral ground-state electron configuration of Thallium (Z = 81) is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹. Across its 6 occupied principal shells (n = 1 to n = 6), the electrons per shell are 2, 8, 18, 32, 18, 3, with 3 valence electrons participating in bonding.
What are the common ion charges and electronegativity of Thallium?
Thallium commonly forms +1, +3 oxidation states or ion charges in compounds. Its Pauling electronegativity is 1.62 and its first ionization energy is 589 kJ/mol.
When was Thallium discovered and what is it used for?
Thallium was discovered in 1861 (Greek thallos — green shoot). Electronics, optics, research
How does Thallium compare to neighboring elements?
Other post-transition metal elements: