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| a chattering or flighty, light-headed person. |
| a printed punctuation mark (‽), available only in some typefaces, designed to combine the question mark (?) and the exclamation point (!), indicating a mixture of query and interjection, as after a rhetorical question. |
| at3 | |
| —the internet domain name for | |
| Austria | |
| At | |
| —the chemical symbol for | |
| 1. | astatine |
| —symbol for | |
| 2. | Also: A ampere-turn |
| AT | |
| —abbreviation for | |
| attainment target | |
At
The symbol for the element astatine.
at- pref.
Variant of ad-.
| astatine (ās'tə-tēn') Pronunciation Key
Symbol At 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. |
| At
The symbol for astatine. |
| aT attotesla |
At
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AT
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At
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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