Answer : The complete alpha decay reaction will be,
[tex]_{292}^{90}\textrm{Th}\rightarrow _{88}^{288}\textrm{Ra}+_2^4He[/tex]
Explanation :
Alpha decay : In this process, alpha particles is emitted when a heavier nuclei decays into lighter nuclei. The alpha particle released has a charge of +2 units.
The general representation of alpha decay reaction is:
[tex]_Z^A\textrm{X}\rightarrow _{Z-2}^{A-4}Y+_2^4\alpha[/tex]
From the given reaction we conclude that,
[tex]Z-2=88\\\\Z=88+2=90\\\\A-4=288\\\\A=288+4=292[/tex]
So, the element is thorium that has atomic number 90 and atomic mass 292.
The complete alpha decay reaction will be,
[tex]_{292}^{90}\textrm{Th}\rightarrow _{88}^{288}\textrm{Ra}+_2^4He[/tex]
