Chemical reactions involve reorganization of the atoms-changes in the way they are bound together. Based on third postulate, atom ratio is fixed, so mass must be constant. James Chadwick described work of Irene Joliet-Curie as neutron emission rather than a new radioactive particle. Niels Bohr -Spectroscopy working with the magnetic spectrum wanted to describe the exact location of the electron. Masses of atoms Gives the relative mass of an atom.
Periodic table is made up of isotopic masses.
Any comments will be appreciated.