X rightarrow224 Ra alpha228 Th230 Th235 U234 U X rightarrow

X rightarrow^224 Ra + alpha^228 Th^230 Th^235 U^234 U X rightarrow^207 Pb + e^-^204 Tl^207 Tl^206 Pb^208 Pb

Solution

part A,

mass on LHS=224 +4(alpha) = 228

atomic number = 88(radium) + 2(alpha) = 90 thorium

option a is correct.

part b)

mass on LHS = 207+ e = 207

atomic number = 82(Pb) -1(e) = 81 (Tl)

option 2 is correct.