A strip of magnesium is placed into a test tube with hydrochloric acid to form magnesium chloride and hydrogen. The mass of the reactants is measure to be 12g. After the reaction is complete, the mass of the system is 11.8g. Knowing the Law of the Conservation of Mass, what happened to the rest of the mass?

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.