Astatine
Astatine is a very rare radioactive chemical element with the chemical symbol At and atomic number 85. It occurs on Earth as the decay product of various heavier elements. All its isotopes are short-lived; the most stable is astatine-210, with a half-life of 8.1 hours.
Identity
- Atomic mass
- [210] u
- Category
- Halogens
- Group
- 17
- Period
- 6
- Block
- p
- Oxidation States
- −1, +1, +3, +5, +7
- Phase (25 °C)
- Solid
- Appearance
- unknown, probably metallic
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p5
- Short configuration
- [Xe] 4f14 5d10 6s2 6p5
- Electrons per shell
- 2, 8, 18, 32, 18, 7
- Electronegativity (Pauling)
- 2.20
- Electron affinity
- 233 kJ/mol
- 1st ionization energy
- 899 kJ/mol
Physical properties
- Melting point
- 301.9 °C (575 K)
- Boiling point
- 336.9 °C (610 K)
- Density
- 6.35 g/cm³
- Molar heat capacity
- —
Chemical behavior of astatine
Astatine is a halogen of period 6 (group 17) that occupies a singular position by combining its own characteristics with a more metallic behaviour than its lighter homologues. Its estimated electronegativity of 2.2 (Pauling scale) is lower than that of iodine, and its high atomic mass influences its electronic structure. Its predominant bond is covalent, although as atomic mass increases, ionic character tends to appear. Exceptions: astatine can form positive species under certain chemical conditions, something uncommon among halogens.
Reactivity
2 At + Cl₂ → 2 AtCl
Astatine reacts with chlorine to give astatine chloride, a compound with ionic character but covalent character, showing the tendency of heavy halogens to form bonds with more electronegative elements.
2 At + H₂O → AtH + O₂ (with oxidant), AtO⁻
With water and oxidising agents, astatine forms species such as astatine hypohalite ion (AtO⁻) and other compounds of +1 or +5 oxidation state.
At₂ + 2 NaOH → NaAt + NaAtO + H₂O
Astatine reacts with hot strong base, forming astatine salts and sodium astatate, showing astatine's amphoteric behaviour in basic medium.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Astatine chloride | AtCl | Stable inorganic salt; model for understanding the chemistry of heavy halogens |
| Astatine bromide | AtBr | Thermally stable compound; precursor of organic astatine compounds |
| Hypoastatous acid | HAtO | Species in the +1 oxidation state; model for studying astatine oxidising chemistry |
| Potassium astatate | KAt | Stable inorganic salt; candidate radiotracer in nuclear medicine |
Electron shell distribution
View in the atom builderIsotopes
Astatine has no stable isotopes: all its isotopes are radioactive, with half-lives ranging from hours to minutes. The most stable known isotope is astatine-210 (²¹⁰At), with a half-life of about 8.1 hours. Astatine-211 (²¹¹At), with a half-life of 7.2 hours, is the most relevant isotope for nuclear medicine due to its emission of alpha particles. Astatine-212 (²¹²At) has a half-life of 0.31 hours. The scarcity and radioactivity of astatine isotopes make it an extremely difficult element to study in macroscopic amounts.
Occurrence and production
Astatine is not found free in nature: its radioactive isotopes are produced in minute amounts as a result of the decay of neptunium and uranium. Its natural abundance in the Earth's crust is less than 1 part in 10^26. Astatine accumulates in certain alloys of well holes and in uranium minerals. It is obtained as a byproduct of nuclear fission and the decay of astatine-210, which is generated from neptunium-237. The main sources of radioactive astatine are nuclear reactors and particle accelerators for the production of isotopes such as astatine-211, used in targeted alpha therapy.
History and origin of the name
Astatine was discovered in 1940 by the American chemists Dale R. Corson, K. R. MacKenzie and Emilio Segrè at the University of California, Berkeley. They bombarded mercury (Hg) with alpha particles in a particle accelerator, observing the decay of astatine-211. The name comes from the Greek 'astatos' (unstable, long-lasting), although it is an incorrect name because astatine isotopes are all radioactive with short half-lives. It was the last halogen discovered before element 117 (tennessine) and before the 1950s.
Applications and relevance of astatine
Uses of the element
Astatine has no known commercial applications due to its radioactivity and scarcity. However, its chemistry is actively studied for its possible use in nuclear medicine, especially in targeted alpha therapy with astatine-211. Radioactive labelling studies of organic compounds with astatine are of interest in the development of targeted drugs and imaging diagnostics.
Uses of its compounds
Astatine compounds have limited applications due to their radioactivity, but astatine-211 is studied as a radiotracer in targeted alpha therapy. The ability of astatine compounds to transport alpha particles to specific tumour cells makes them candidates for cancer treatments in advanced stages of clinical development. Astatine chemistry is fundamental for designing molecules that can stably transport this radionuclide to the target tissue.
Health, safety and environment
Biological role
Astatine is highly radioactive and its toxicity is greater than that of iodine due to its radioactivity. Like iodine, astatine incorporates into the thyroid and other structures, which can cause damage to thyroid tissue. However, its radioactivity makes astatine dangerous even in very small doses. Study of its biology is based on animal models and theoretical calculations. The radiation dose received depends on the amount of astatine ingested and the duration of its retention in the body.
Safety
Astatine is highly radioactive and requires handling in high-security facilities, with strict radiation exposure control, trained personnel and appropriate personal protective equipment. Astatine samples must be handled in sealed systems, with radiation detection and specialised safety procedures. Astatine produces radiolysis of water and can generate reactive species that, in addition to radioactivity, can be dangerous. Radioactive waste must be treated according to applicable radiological safety regulations.
Environment
Astatine is not present in the natural environment in significant quantities, and its radioactivity limits its environmental presence. Any astatine released into the environment rapidly decays radioactively and does not accumulate in the food chain. Environmental studies of astatine focus on radioactive waste control and the safety of medical facilities that use astatine-211.
Frequently asked questions about Astatine
Why does astatine have properties of both halogen and metal?
Astatine occupies a singular position because it combines its own halogen characteristics with a more metallic behaviour than its lighter homologues. Under certain chemical conditions it can form negative species, like other halogens, but also relatively stable positive species. This duality is related to its high atomic mass and to relativistic effects that influence its electrons. Its study allows us to understand how chemical properties change as we move to heavier elements of the periodic table.
What is astatine-211 and why is it interesting for nuclear medicine?
Astatine-211 is a radioactive isotope that emits alpha particles, capable of depositing a large amount of energy in very short distances. This characteristic makes it a candidate for targeted alpha therapy, a strategy that seeks to transport the radionuclide to tumour cells through molecules designed to recognise them. The aim is to concentrate the radiation effect on the target tissue and limit exposure of nearby tissues. Its development requires solving challenges related to production, chemical labelling and stability of compounds.
How can astatine form bonds with organic molecules?
Astatine can incorporate into certain organic molecules through bonds between the astatine atom and carbon-based structures. This possibility is especially important for developing radiolabelled molecules that can recognise specific biological targets. However, the bond must be sufficiently stable to prevent astatine from separating from the molecule before reaching its destination. Researchers study different labelling strategies and bond types to improve the chemical stability of compounds and facilitate their possible application in nuclear medicine.
Can astatine form stable chemical compounds?
Astatine can form different chemical compounds, both inorganic and organic, but their stability depends on molecular structure and environmental conditions. The main experimental problem is that the radioactive atom itself decays, so it cannot be studied as a persistent element in a conventional sample. In addition, some bonds that appear chemically suitable may break under certain conditions. Understanding these differences is fundamental for investigating its chemistry and designing more stable radiolabelled molecules.
Why does astatine behave differently from iodine?
Although astatine and iodine belong to the same chemical family, astatine has important differences due to its electronic structure and relativistic effects. Its behaviour may be less typically non-metallic, and some of its positive species show characteristics that are not usual among lighter halogens. These differences affect bond formation and the stability of its compounds. However, experimental knowledge is still limited, so some properties are still interpreted through theoretical models and chemical comparisons.
Why is it so difficult to know the physical properties of astatine?
The study of astatine is limited because its isotopes are radioactive and can only be handled in extremely small amounts. It is not possible to prepare macroscopic samples that allow direct measurement of many properties of materials that are usual with more stable elements. For this reason, researchers resort to radioactive tracer techniques, very low-concentration chemical experiments and theoretical calculations. As a result, some physical properties attributed to astatine are predictions or estimates, and not direct observations of a visible sample.
What are relativistic effects and why are they important in astatine?
Relativistic effects appear when the high nuclear charge has an important influence on the movement and energy of inner electrons. In heavy elements, these modifications also affect outer electrons and can change orbital energies, bond formation and the stability of certain chemical species. In astatine, these effects help explain why its behaviour does not exactly follow the trends observed in fluorine, chlorine, bromine and iodine. They are essential for interpreting its chemistry.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.