What particle is needed to complete this nuclear reaction? 222 86 rn 218po?

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