Tonium (T) has a molar mass of 210.0 g/mol and Changston (Ch) has a molar mass of 55.0 g/mol. The molar mass of TCh2 is 320.0g/mol. What is the percent by
mass of T?

the answer is na (sodium)

considering the question as per the attachment

here we can see that the radon nuclide has 222 as the atomic mass number and 86 as the atomic number.

when it undergoes nuclear fission it gives polonium which has atomic mass number as 218 and atomic number will be 84 along with a helium nucleus which has 4 atomic mass number and 2 as its atomic number.

222 = 218 + 4 (atomic mass number)

86 = ? + 2; therefore ? = 86-2 = 84.

the atomic mass number of polonium will be 84.