Answer: The mass number of X is 218.
Explanation:
Alpha decay : When a larger nuclei decays into smaller nuclei by releasing alpha particle. In this process, the mass number and atomic number is reduced by 4 and 2 units respectively.
General representation of an element is given as: [tex]_Z^A\textrm{X}[/tex]
where,
Z represents Atomic number
A represents Mass number
X represents the symbol of an element
General representation of alpha decay :
[tex]_Z^A\textrm{X}\rightarrow _{Z-2}^{A-4}Y+_2^4\alpha[/tex]
[tex]_{86}^{222}\textrm{Rn}\rightarrow _{84}^{218}\textrm{Po}+_2^4\textrm{He}[/tex]
