Polonium-210 has a Atomic mass of 210 g/mol. The reaction of decay is shown as,
²¹⁰₈₄Po ---------> ᵇₐX + ⁴₂He
where,
X = Unknown Element
ᵇ = Atomic Mass of Unknown Element
ᵃ = Atomic number of Unknown Element
Solving for ᵇ,
210 = ᵇ + 4
ᵇ = 210 - 4
ᵇ = 206
Solving for ₐ ,
84 = ₐ + 2
ₐ = 84 - 2
ₐ = 82
Now, Putting values in ᵇ and ₐ of X,
²⁰⁶₈₂X
Searching Periodic table for atomic number 82.
²⁰⁶₈₂Pb
Result: The child Isotope is Lead (Pb) having atomic number 82 and Atomic mass 206 g/mol.