[tex]^{222}_{\phantom{2}86}\text{Rn} \to ^{218}_{\phantom{2}84}\text{Po}+ ^{4}_{2}\text{He}[/tex]
Y is Po-218.
[tex]^{222}_{\phantom{2}86}\text{Rn} \to ^{A}_{Z}\text{Y}+ ^{4}_{2}\text{He}[/tex]
Here's the symbol of a particle in a nuclear reaction. [tex]^{A}_{Z}\text{Y}[/tex].
A stands for mass number. Z stands for atomic number. Both numbers shall conserve in a nuclear reaction.
So is the case for the atomic number. Try figure out the atomic number of Y using the same approach.
What element is Y? The atomic number of Y is 84. Refer to a periodic table. Element 84 corresponds to Po (polonium). Y is Polonium-218. The symbol of Y should be written as [tex]^{218}_{\phantom{2}84}\text{Po}[/tex]. Hence the equation:
[tex]^{222}_{\phantom{2}86}\text{Rn} \to ^{218}_{\phantom{2}84}\text{Po}+ ^{4}_{2}\text{He}[/tex].