Chemical element · reference page

85At

Astatine

Astatine is element 85 (At), located in period 6, group 17 and the p block. At room temperature its reference phase is solid. Its electron configuration is [Xe] 4f14 5d10 6s2 6p5.

Atomic number85

Standard atomic weightNo standard atomic weight

Group17

Period6

Blockp

Electron configuration[Xe] 4f14 5d10 6s2 6p5

Electronic structure

What controls its chemistry

Halogens have seven valence electrons. Gaining one electron completes the outer shell, so −1 halide ions are common, while covalent halogen compounds cover a wide range of oxidation states for the heavier members.

The free elements are reactive and are generally not found uncombined in nature. Reactivity and physical state change down the group as atoms become larger and more polarizable.

Chemical forms

Compounds, ions and materials

The free element is molecular or otherwise highly reactive, while halide ions are among the most common chemical forms. Hydrogen halides, metal halides, interhalogen compounds and, for the heavier halogens, oxygen-containing compounds broaden the chemistry well beyond the simple −1 state.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The melting point is 575 K (301.9 °C). The boiling point is 610 K (336.9 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where astatine is found and used

In practice, astatine is encountered in scientific research, nuclear medicine and chemical synthesis.

In practice

Examples of real uses

Scientific research

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

Nuclear medicine

A suitable astatine radioisotope can be used for imaging or treatment when its radiation type and half-life match the medical task.

Chemical synthesis

Astatine is used as an element, reagent, catalyst component or precursor in the manufacture of other chemicals.

Periodic position

What its neighbours tell us

Astatine is in period 6 and group 17. In the neighbourhood shown here, it lies between polonium and radon in the same period; iodine is above it and tennessine below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationmetalloid

Phase at room temperatureSolid

Electron shells2 · 8 · 18 · 32 · 18 · 7

Density—

Melting point575 K (301.9 °C)

Boiling point610 K (336.9 °C)

Pauling electronegativity2.2

First ionisation energy899 kJ/mol

Electron affinity233 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 astatine is associated with Dale R. Corson. 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

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

Astatine 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

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