Hi, thank you for posting your question here at Brainly.
The element here is Radon (Rn) with atomic number of 86. To solve this. add the mass of an electron to the isotopic mass (due to electron capture). Then, from here, subtract the corresponding mass of gamma radiation. (931 mev = 1 amu)
*mass of electron = 0.00054858 amu
isotopic mass = (209.989669 amu + 0.00054858 amu) - 2.368 mev*(1 amu/931 mev)
isotopic mass = 209.9876741 amu
