Antoine Lavoisier made chemicals react within a closed container. The mass of the container and chemicals after the reaction weighed the same as the container and chemicals before the reaction showing that no mass was lost. This led to the Law of . Action-Reaction

Elasticity

Atoms

Conservation of Mass

Conservation of Mass

Explanation:

I did the test on K12

Conservation of Mass

Explanation:

Lavoisier conducted many experiments of this type. He found in all cases that the mass of the reactants is equal to the mass of the products. His conclusion, called the states that in a chemical reaction, atoms are neither created nor destroyed. - www.classzone.com

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.