| 单词 | protactinium |
| 释义 | Word Frequency protactinium(ˌprəʊtækˈtɪnɪəm) noun a toxic radioactive metallic element that occurs in uranium ores and is produced by neutron irradiation of thorium. Symbol: Pa; atomic no: 91; half-life of the most stable isotope,231Pa: 32 500 years; valency: 4 or 5; relative density: 15.37 (calc.); melting pt: 1572°C Former name: protoactinium |
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