atoms of different elements vary in size and mass
what are the relative masses of each different kind of atom?
An inquiry into the relative weights of the ultimate particles of bodies is a subject
In the 1800s, John Dalton was the first scientist to explain the behavior of atoms in terms of the measurement of weight.
Dalton based his theory on the law of conservation of mass and the law of constant composition. The first part of his theory states that all matter is made of atoms, which are indivisible.
Following it states all atoms of a given element are identical in mass and properties, compounds are combinations of two or more different types of atoms, and that a chemical reaction is a rearrangement of atoms.