# 238^92_U is known to undergo radioactive decay to form 206^82_Pb by emitting alpha and beta particles. A rock initially

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$_{92}^{238}U$ is known to undergo radioactive decay to form $_{82}^{206}Pb$ by emitting alpha and beta particles. A rock initially contained 68 × 10–6 g of $_{92}^{238}U$ . If the number of alpha particles that it would emit during its radioactive decay of $_{92}^{238}U$ to $_{82}^{206}Pb$ in three half-lives is Z × 1018 , then what is the value of Z ?

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In decay from U238 to Pb206, each U238 atom decays and produces 8 $\alpha$-particles and hence, total number of $\alpha$-particles emitted out

= 1.204 x 1018