Final answer:
The particle or nucleus X needed to complete the equation is 84^218Po.
Step-by-step explanation:
Radon has an atomic number of 86, so the parent isotope is represented as 222Rn. We represent the alpha particle as He and use subtraction (222 − 4 = 218 and 86 − 2 = 84) to identify the daughter isotope as an isotope of polonium, 218Po: Therefore, the completed nuclear decay equation is 86210Rn -> 84206Po + 24He. After the alpha decay, polonium-206 remains as the daughter nucleus.