Fermium

Fermium is element 100 (Fm), located in period 7, group — and the f block. At room temperature its reference phase is solid. Its electron configuration is [Rn] 5f12 7s2.

Atomic number100

Standard atomic weightNo standard atomic weight

Group—

Period7

Blockf

Electron configuration[Rn] 5f12 7s2

Electronic structure

What controls its chemistry

Actinide chemistry involves the 5f, 6d and 7s orbitals. The early actinides can access several oxidation states; for the heavier members, +3 becomes increasingly important.

Every actinide is radioactive. Nuclear stability and isotope half-life are therefore inseparable from the chemistry, especially for elements that exist only in trace quantities or must be produced artificially.

Chemical forms

Compounds, ions and materials

Actinides form oxides, halides and coordination compounds, with several oxidation states accessible for the early members. The chemistry cannot be separated from radiochemistry because every isotope is radioactive and sample composition can change through decay.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The melting point is 1 125 K (851.9 °C). Where a phase-transition value is not listed, the behaviour may be poorly defined under ordinary pressure, depend strongly on allotrope, or lack a reliable reference value. Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where fermium is found and used

In practice, fermium is encountered in scientific research.

In practice

Examples of real uses

Scientific research

Fermium 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

Fermium is in period 7. In the f-block sequence shown here, it appears between einsteinium and mendelevium. This sequence allows a direct comparison with the neighbouring f-block elements.

Reference data

Physical and atomic properties

Classificationactinide

Phase at room temperatureSolid

Electron shells2 · 8 · 18 · 32 · 30 · 8 · 2

Density—

Melting point1 125 K (851.9 °C)

Boiling point—

Pauling electronegativity1.3

First ionisation energy627 kJ/mol

Electron affinity—

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 fermium is associated with Lawrence Berkeley National Laboratory. 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

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

Fermium 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

Fermium is radioactive. Work with it requires radiological controls appropriate to the isotope, activity, radiation type and chemical form.