How does the law of conservation of mass relate to the atomic theory?
At the atomic and nuclear scale, the law of conservation of mass strictly doesn't hold.
Mass and energy are treated as inter-convertible entities and the net value of mass-energy is conserved.
While studying nuclear physics, you should come up with plenty of examples where the inter-conversion of matter (nucleons) and energy is important.
Such observations cannot be explained in terms of the law of conservation of mass and conservation of energy.
The mass-energy equivalence is given by Einstein's most famous equation :