Question

Which step in the decay scheme explains how ${}^{210}Pb$ ends up in rain water while its parent ${}^{238}U$ is only present in earth crust?

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

Answer 1

Answer: (D)

${}^{226}Ra{-}^{222}Rn$

Radium 226 produces radon 222 on decaying. Radon gas can escape to the atmosphere before it decays and after a residence time, it decays to polonium 218 which falls to the surface of the earth with dust and rain. Eventually, lead 210 is produced and can thus end up in rainwater.