Alpha decay is defined as emission of alpha particle which is [tex]^4_{2}He[/tex] from the parent nuclei to form daughter nuclei.
Here, parent nuclei is [tex]^{210}_{84}Po[/tex] with atomic mass 210 and atomic number 84. Similarly for alpha particle, atomic mass is 4 and atomic number is 2. After emission of alpha particle, atomic number of Po-210 get reduced by 2 and atomic mass gets reduced by 4. The resultant atomic number and atomic mass will be 82 and 2016 respectively. This represent the isotope of lead, [tex]^{206}_{82}Pb[/tex].
The reaction will be as follows:
[tex]^{210}_{84}Po\rightarrow ^{206}_{82}Pb+^{4}_{2}He[/tex]
Therefore, the daughter nuclei will be [tex]^{206}_{82}Pb[/tex].
