Thallium (Tl): Electron Configuration, Properties and Uses
Thallium (Tl): Electron Configuration, Properties and Uses
Electron configuration [Xe] 6s2 4f14 5d10 6p1 · group 13 · period 6 · Ar 204.38
Thallium data
- Atomic number
- 81
- Relative atomic mass
- 204.38
- Group
- 13
- Period
- 6
- Block
- p
- Category
- Post-transition metal
- Electron configuration
- [Xe] 6s2 4f14 5d10 6p1
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p1
- Electrons per shell
- 2, 8, 18, 32, 18, 3 (K L M N O P)
- Valence electrons
- 3
- Electronegativity (Pauling)
- 1.62
- First ionisation energy
- 6.108 eV
- Atomic radius (van der Waals)
- 196 pm
- State at room temperature
- Solid
- Melting point
- 304 °C
- Boiling point
- 1473 °C
- Density
- 11.8 g/cm³
- Common oxidation states
- +3, +1
- Discovered
- 1861
Source: PubChem (NIH, accessed 13 Nov 2026); atomic mass: IUPAC standard atomic weight.
Electron shell diagram
K 2 · L 8 · M 18 · N 32 · O 18 · P 3
Orbital diagram (outer electrons)
About thallium
Thallium is a chemical element with the symbol Tl and atomic number 81. It is a post-transition metal in group 13, period 6, p-block. The element was discovered in 1861. Thallium is a solid at room temperature, melting at 304 °C and boiling at 1473 °C.
Thallium electron configuration
The electron configuration of thallium is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p1, or in short [Xe] 6s2 4f14 5d10 6p1. Its electrons occupy 6 shells: K = 2, L = 8, M = 18, N = 32, O = 18, P = 3. Thallium has 3 valence electrons in the P shell (6s2 6p1), which is why it sits in group 13.
Filling order: 1s2(2) → 2s2(4) → 2p6(10) → 3s2(12) → 3p6(18) → 4s2(20) → 3d10(30) → 4p6(36) → 5s2(38) → 4d10(48) → 5p6(54) → 6s2(56) → 4f14(70) → 5d10(80) → 6p1(81)
Uses of thallium
The main uses of thallium include infrared detectors and high-refractive-index glass; Tl-201 was formerly used for heart imaging.
Safety
Thallium and its compounds are highly toxic and can be absorbed through the skin. Always read the safety data sheet (SDS) before working with a chemical.
Other elements in group 13
Frequently asked questions
What is the electron configuration of thallium?
The electron configuration of thallium (Tl, atomic number 81) is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p1, or [Xe] 6s2 4f14 5d10 6p1 in short. By shell: 2, 8, 18, 32, 18, 3.
How many valence electrons does thallium have?
Thallium has 3 valence electrons.
Which group and period is thallium in?
Thallium is in group 13, period 6, p-block.
What is the atomic mass of thallium?
The relative atomic mass of thallium is 204.38. Use it to calculate molar mass with the molar mass calculator.
Prepare solutions with analytical-grade reagents
AMI Scientific is one of the official TCI distributors in Indonesia. Reagents with lot CoA & SDS, formal quotations and tax invoices for universities and laboratories.
Other formulas & calculators: molarity, mole, molality, normality, dilution, titration, pH, buffer, molar mass, ppm, mole fraction, mass percent, periodic table, chemical MSDS.
