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Promethium is a rare radioactive element (symbol Pm, atomic number 61) that is extremely scarce in nature, with only about 500–600 grams existing in Earth's crust at any given time. It's notable for being one of only two radioactive elements sandwiched between stable elements on the periodic table, and it belongs to the lanthanide family of elements.
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Discovered by embedding cosine similarity (sentence-transformers MiniLM, 384-dim).