If two samples have different elemental composition, they are different compounds. He connected Proust's law with the concept of atoms to deduce that if elements were composed of tiny individual particles, a given compound should always contain the same combination of these atoms. Clearly these are different compounds. Chemical compounds are formed when atoms combine. A given compound has the same relative numbers and types of atoms.
Chemical reactions involve re-organization of the atoms, the way they are bound together.
The atoms themselves are not changed. In a chemical reaction at the same temperature and pressure, the same volume of gases always reacts. This implies that the distances between particles in the gas is much larger than the size of the particles themselves. A bit more about modern tools . Working much like a miniature stylus profilometer, the Park Scientific autoprobe AFM images topography to subnanometer scales.