Radon is a radioactive gas. It is colorless, odorless, tasteless, and chemically inert. Unless you test for it, there is no way of telling how much is present. Radon is formed by the natural radioactive decay of uranium in rock, soil, and water.

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Reactivity

Common on earth%

Radon is rare in nature because it's an isotopes are short-lived and because it's source, radium, is a scarce element. The common earth percent is 0% in universe.