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countable and uncountable, plural astatines
A highly radioactive chemical element (symbol At), one of the halogens, with atomic number 85. quotations examples
Astatine, for instance, is practically unstudied. It has a name and a place on the periodic table (next to Marie Curie's polonium), but almost nothing else.
2003, Bill Bryson, A Short History of Nearly Everything, BCA, page 221