Chemical element · reference page

61Pm

Promethium

Promethium is element 61 (Pm), located in period 6, group — and the f block. At room temperature its reference phase is solid. Its electron configuration is [Xe] 4f5 6s2.

Atomic number61

Standard atomic weightNo standard atomic weight

Group—

Period6

Blockf

Electron configuration[Xe] 4f5 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 7.26 g/cm³. The melting point is 1 315 K (1 042 °C). The boiling point is 3 273 K (3 000 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where promethium is found and used

In practice, promethium is encountered in nuclear batteries, scientific research and luminous paint.

In practice

Examples of real uses

Nuclear batteries

Radioactive decay of promethium can be converted into long-lived electrical power in specialised devices.

Scientific research

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

Luminous paint

Radioactive or luminescent compounds have been used historically to make markings visible in the dark; many older formulations are no longer acceptable.

Periodic position

What its neighbours tell us

Promethium is in period 6. In the f-block sequence shown here, it appears between neodymium and samarium. 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 · 23 · 8 · 2

Density7.26 g/cm³

Melting point1 315 K (1 042 °C)

Boiling point3 273 K (3 000 °C)

Pauling electronegativity—

First ionisation energy539kJ/mol

Electron affinity12.45 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 promethium is associated with Chien Shiung Wu. 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

Promethium has no standard atomic weight. There is no characteristic terrestrial isotopic mixture from which a stable abundance-weighted average can be assigned.

Promethium has no stable isotope. Its isotopes are radioactive, and their half-lives and decay modes become important whenever the element is measured, handled or used.

Safety

Handling and exposure

The hazards associated with promethium depend on chemical form, dose and route of exposure. Pure promethium, 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.