Each new theory is an advancement of an earlier theory.
In science, each researcher builds on existing knowledge in his/her area of research. A new researcher often gathers all existing information in his /her area of research before developing his/her research hypothesis. The new research hypothesis charts a new course in the search for knowledge and broadens the horizon of knowledge in that discipline.
J.J Thompson postulated a plum pudding model of the atom. His model already entrenched the idea of the existence of subatomic particles in our minds. This is an advancement over Dalton's theory of the 'indivisible' atom. Rutherford proposed a planetary model of the atom. His model exposed us to the idea of the relative positions of subatomic particles within the atom. This is an advancement over Thompson's original proposition. Neil's Bohr applied the emerging theory of quantum mechanics to his model to explain the spectral lines in the hydrogen atom. This was a huge milestone in understanding the actual nature of the atom. The wave mechanical model takes into account the Heisenberg uncertainty principle and the wave-particle paradox in its approach to the arrangement of electrons in an atom. This approach gives a better perspective to the study of atomic structure than earlier models.
Hence each model gives a better picture of the previous models without utterly discrediting it. Therefore knowledge of atomic structure advanced with each new model being an improvement of earlier models.
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.