Chemical element · reference page

69Tm

Thulium

Thulium is element 69 (Tm), located in period 6, group — and the f block. At room temperature its reference phase is solid. Its electron configuration is [Xe] 4f13 6s2.

Atomic number69

Standard atomic weight168.934219(5)

Group—

Period6

Blockf

Electron configuration[Xe] 4f13 6s2

Electronic structure

What controls its chemistry

The lanthanides are characterised by filling of the 4f subshell. Their chemistry is often dominated by the +3 oxidation state, while partially filled f orbitals give many members useful magnetic and optical properties.

Because the 4f electrons are relatively shielded, neighbouring lanthanides have very similar chemical behaviour. Separating them from one another is therefore much harder than separating elements from different main groups.

Chemical forms

Compounds, ions and materials

Lanthanides are commonly encountered as +3 ions in oxides, halides and coordination compounds. Their compounds often have closely related chemistry, while differences in f-electron structure create distinctive magnetic and luminescent behaviour.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 9.32 g/cm³. The melting point is 1 822 K (1 549 °C). The boiling point is 2 223 K (1 950 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where thulium is found and used

In practice, thulium is encountered in lasers, xray sources and scientific research.

In practice

Examples of real uses

Lasers

Thulium is used in laser media, optical components or wavelength-specific light generation.

Xray sources

Thulium is used in compact radiation sources or target materials for specialised X-ray generation.

Scientific research

Thulium is produced, isolated or measured mainly to test atomic, nuclear and periodic trends rather than for bulk commercial use.

Periodic position

What its neighbours tell us

Thulium is in period 6. In the f-block sequence shown here, it appears between erbium and ytterbium. This sequence allows a direct comparison with the neighbouring f-block elements.

Reference data

Physical and atomic properties

Classificationlanthanide

Phase at room temperatureSolid

Electron shells2 · 8 · 18 · 31 · 8 · 2

Density9.32 g/cm³

Melting point1 822 K (1 549 °C)

Boiling point2 223 K (1 950 °C)

Pauling electronegativity1.25

First ionisation energy596.7 kJ/mol

Electron affinity99 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of thulium is associated with Per Teodor Cleve. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is 168.934219(5). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Thulium has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The hazards associated with thulium depend on chemical form, dose and route of exposure. Pure thulium, its ions and its compounds can have very different biological and environmental effects, so safety data should be checked for the actual substance being handled.