An element composed of tiny particles is called atoms. Atoms of different elements have different properties. In an ordinary chemical reaction, no atom of any element disappears of is changed into an atom of another element. Compounds are formed when atoms of two or more elements combine. In a given compound, the relative numbers of atoms of each kind are definite and constant whole number ratios.
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.