Question

Which step in the decay scheme explains how Pb-219 ends up in rain water while its parent U-238 is only present in earth's crust?

• A. ${}^{238}U{-}^{234}U$
• B. ${}^{234}U{-}^{230}Th$
• C. ${}^{230}Th{-}^{226}U$
• D. ${}^{226}Ra{-}^{222}Rn$

Answer 1

The correct option is D ${}^{226}Ra{-}^{222}Rn$

The step ${}^{226}Ra{-}^{222}Rn$ in the decay scheme explains how ${}^{219}Pb$ ends up in rain water while its parent ${}^{238}U$ is only present in earth's crust.

${}^{222}Rn$ is a gas and enters earths atmosphere through which it enters rain water.