Astatine – facts and properties
- Symbol
- At
- Atomic number
- 85
- Atomic mass
- 210 u
- Period
- 6
- Group
- 17
- Category
- halogen
- Electron configuration
- [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵
- Electronegativity
- 2.2
- Atomic radius
- 127 pm
- Valence electrons
- 7
- State at standard conditions
- solid
- Melting point
- 302 °C
- Density
- 7 g/cm³
- Discovered
- 1940
Astatine is a radioactive halogen with atomic number 85 and the rarest naturally occurring element. At any moment, the whole of the Earth's crust is estimated to hold less than a gram of it. All of its isotopes decay within hours, which is why the name comes from the Greek astatos, meaning unstable. Astatine was first made in 1940 in California. Today the isotope astatine-211 is being tested in research on targeted cancer treatment.
Position in the periodic table
Occurrence and extraction
In nature, astatine arises only as a rare side reaction of the decay series of uranium and thorium. Some radioactive nuclei do not only change into the next member of the series but, in a tiny fraction of cases, into astatine isotopes. These decay again within fractions of a second to about a minute. Formation and decay balance each other, so the amount stays minute. Astatine is thus rarer than any other element that occurs naturally. Berta Karlik and Traude Bernert in Vienna traced it in these decay series in the early 1940s.
Years earlier, researchers had announced the discovery of the missing halogen below iodine, but their findings could not be confirmed. The proof came in 1940 at the University of California in Berkeley, where Dale Corson, Kenneth MacKenzie and Emilio Segrè bombarded bismuth with alpha particles in a cyclotron and obtained astatine-211. They chose the name because of the element's instability.
Properties and reactions
In the periodic table astatine sits in period 6 and group 17: below iodine and between polonium on the left and radon on the right. It has 7 valence electrons, and its electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵. Its atomic mass is given as 210 u, the mass number of the longest-lived isotope, astatine-210, whose half-life is about eight hours.
Almost none of its physical properties have been measured. The melting point of 302 °C, the density of 7 g/cm³ and the electronegativity of 2.2 are estimates, derived from trends in the periodic table and from calculations. No value is available for the boiling point. Astatine is probably a dark solid, and whether it behaves more like a metal or a semimetal is unsettled.
A visible amount has never been made. Its radiation would heat it so strongly that it would vaporise at once. Studies are therefore carried out on invisibly small traces in solution.
Chemically astatine resembles iodine but is more metallic. It forms astatide ions, At⁻, and compounds with iodine and with carbon. Like iodine, it collects in the thyroid gland in the body.
Research and synthesis
Astatine is not obtained from ores but made artificially in particle accelerators. As in its discovery, bismuth is bombarded with alpha particles in a cyclotron. Since astatine-211 has a half-life of only about seven hours, it has to be processed immediately afterwards. Only a few cyclotrons in the world deliver alpha beams of suitable energy.
There is no everyday use, but there is promising research. Astatine-211 emits alpha particles that reach only a few cell layers into tissue. Targeted alpha therapy exploits this: astatine-211 is attached to antibodies or other molecules that recognise tumour cells. The radiation then acts mainly in the tumour and spares the healthy tissue around it. First clinical trials are under way, for example on brain tumours and blood cancer. The method still belongs to research and is not standard treatment.
Quick question
What is the atomic number of astatine (At)?
FAQ
Why is astatine so rare?
Astatine appears only as a tiny side branch of the decay series of uranium and thorium, and its isotopes decay again within hours. That is why the whole Earth's crust holds less than a gram.
Where does the name astatine come from?
It goes back to the Greek astatos, meaning unstable. This refers to its radioactivity: the longest-lived isotope has a half-life of only about eight hours.
Can astatine be seen or weighed?
No. So far only invisibly small amounts have existed. A visible sample would heat itself with its own radiation and vaporise at once, which is why many of its properties are only estimated.