Chemical element · reference page

90Th

Thorium

Thorium is element 90 (Th), located in period 7, group — and the f block. At room temperature its reference phase is solid. Its electron configuration is [Rn] 6d2 7s2.

Atomic number90

Standard atomic weight232.0377(4)

Group—

Period7

Blockf

Electron configuration[Rn] 6d2 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 listed density is 11.70g/cm³. The melting point is 2 020 K (1 747 °C). The boiling point is 5 058K (4 788 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where thorium is found and used

In practice, thorium is encountered in nuclear energy, alloys and scientific research.

In practice

Examples of real uses

Nuclear energy

Thorium is important in nuclear fuel cycles or in research on nuclear energy systems.

Alloys

Thorium is added to metals to change strength, corrosion resistance, melting behaviour or high-temperature performance.

Scientific research

Thorium 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

Thorium is in period 7. In the f-block sequence shown here, it appears between actinium and protactinium. 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 · 18 · 10 · 2

Density11.70g/cm³

Melting point2 020 K (1 747 °C)

Boiling point5 058K (4 788 °C)

Pauling electronegativity1.3

First ionisation energy609kJ/mol

Electron affinity112.7 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 thorium is associated with Jöns Jakob Berzelius. 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 232.0377(4). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Thorium 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

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