Nuclear reaction involved in present case is
222 86 Rn → 218 84 Po + X
Step 1: Balancing Atomic Number:
On reactant side atomic number is 86. On product side atomic number is 84. Hence, atomic number of X is 86 - 84 = 2
Step 2: Balancing atomic mass number:
On reactant side atomic number is 222. On product side atomic number is 218. Hence, atomic number of X is 222-218 = 4
Therefore, element X has atomic number 2 and atomic mass number 4. This corresponds to He.
Hence complete reaction is
222 86 Rn → 218 84 Po + 4 2 He
