|1.||used to indicate location or position: are they at the table?; staying at a small hotel|
|2.||towards; in the direction of: looking at television; throwing stones at windows|
|3.||used to indicate position in time: come at three o'clock|
|4.||engaged in; in a state of (being): children at play; stand at ease; he is at his most charming today|
|5.||(in expressions concerned with habitual activity) during the passing of (esp in the phrase at night): he used to work at night|
|6.||for; in exchange for: it's selling at four pounds|
|7.||used to indicate the object of an emotion: angry at the driver; shocked at his behaviour|
|8.||slang where it's at the real place of action|
|[Old English æt; related to Old Norse at to, Latin ad to]|
|—the internet domain name for|
|—the chemical symbol for|
|2.||Also: A ampere-turn|
|1.||Also: atm atmosphere (unit of pressure)|
The symbol for the element astatine.
The symbol for astatine.
|astatine (ās'tə-tēn') Pronunciation Key
A highly unstable, rare, radioactive element that is the heaviest of the halogen elements. Its most stable isotope has a half-life of 8.3 hours. Atomic number 85; melting point 302°C; boiling point 337°C; valence probably 1, 3, 5, 7. See Periodic Table.
radioactive chemical element and the heaviest member of the halogen elements, or Group 17 (Group VIIa) of the periodic table. Astatine, which has no stable isotopes, was first synthetically produced (1940) at the University of California by American physicists Dale R. Corson, Kenneth R. MacKenzie, and Emilio Segre, who bombarded bismuth with accelerated alpha particles (helium nuclei) to yield astatine and neutrons. Naturally occurring astatine isotopes have subsequently been found in minute amounts in the three natural radioactive decay series, in which they occur by minor branching (astatine-218 in the uranium series, astatine-216 in the thorium series, and astatine-215 and astatine-219 in the actinium series). Thirty-three isotopes are known; astatine-210, with a half-life of 8.3 hours, is the longest lived.
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