Thulium (Tm): Electron Configuration, Properties and Uses
Thulium (Tm): Electron Configuration, Properties and Uses
Electron configuration [Xe] 6s2 4f13 · f-block, lanthanide series · period 6 · Ar 168.93
Thulium data
- Atomic number
- 69
- Relative atomic mass
- 168.93
- Group
- – (lanthanide series)
- Period
- 6
- Block
- f
- Category
- Lanthanide
- Electron configuration
- [Xe] 6s2 4f13
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f13
- Electrons per shell
- 2, 8, 18, 31, 8, 2 (K L M N O P)
- Valence electrons
- 2 (in the outer shell)
- Electronegativity (Pauling)
- 1.25
- First ionisation energy
- 6.184 eV
- Atomic radius (van der Waals)
- 227 pm
- State at room temperature
- Solid
- Melting point
- 1545 °C
- Boiling point
- 1950 °C
- Density
- 9.32 g/cm³
- Common oxidation states
- +3
- Discovered
- 1879
Source: PubChem (NIH, accessed 13 Nov 2026); atomic mass: IUPAC standard atomic weight.
Electron shell diagram
K 2 · L 8 · M 18 · N 31 · O 8 · P 2
Orbital diagram (outer electrons)
About thulium
Thulium is a chemical element with the symbol Tm and atomic number 69. It belongs to the lanthanide series in period 6 (f-block), usually shown below the main table. The element was discovered in 1879. Thulium is a solid at room temperature, melting at 1545 °C and boiling at 1950 °C.
Thulium electron configuration
The electron configuration of thulium is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f13, or in short [Xe] 6s2 4f13. Its electrons occupy 6 shells: K = 2, L = 8, M = 18, N = 31, O = 8, P = 2. As a lanthanide, thulium fills the 4f subshell. Its outer 6s subshell holds 2 electrons; valence electrons for this series are not assigned by the usual group rule. When it forms cations, electrons leave the outermost subshell first: Tm3+ = [Xe] 4f12.
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) → 4f13(69)
Uses of thulium
The main uses of thulium include portable X-ray sources and medical lasers.
Safety
As a solid metal, thulium is not especially hazardous. Fine powder can burn, and its compounds should be handled according to their SDS. Always read the safety data sheet (SDS) before working with a chemical.
Other lanthanides
Frequently asked questions
What is the electron configuration of thulium?
The electron configuration of thulium (Tm, atomic number 69) is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f13, or [Xe] 6s2 4f13 in short. By shell: 2, 8, 18, 31, 8, 2.
How many valence electrons does thulium have?
Thulium has 2 electrons in its outer shell. For lanthanides and actinides, valence electrons are not assigned by the usual group rule.
Which group and period is thulium in?
Thulium is in the lanthanide series, period 6, f-block.
What is the atomic mass of thulium?
The relative atomic mass of thulium is 168.93. Use it to calculate molar mass with the molar mass calculator.
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