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

Anions are negative charged ions