What particle is needed to complete this nuclear reaction 222 86 rn - - - }218 84 po?

To complete this reaction a helium particle must be added.

²²²Rn₈₆ - - > ²¹⁸Po₈₄ + ⁴He₂

This reaction is an example of α decay. In this decay an unstable atom becomes stable by emitting αparticle that is a nuclei a helium so its mass is reduced by 4 units and its atomic number is reduced by 2 units. And the atom becomes stable.

On going from ²²²Rn₈₆ to ²¹⁸Po₈₄, we can see that there is difference in mass number of 4 and in atomic number of 2. So this is clear that a helium nuclei is emitted.