有三个天然存在的放射系列，即钍系、锕系和铀系。
There are three naturally occurring radioactive series - the thorium series, the actinium series, and the uranium series.
这些元素都有放射性，而且性质与锕相似。
All are radioactive and have similar properties to actinium.
锕射气氡的一种放射性的，惰性的和气态的同位素，其半衰期为3。92秒。
A radioactive, inert, gaseous isotope of radon, with a half - life of3.92seconds.
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