The understanding of the nature of matter as we know it today was first presented by John Dalton. His postulates are the basis for all modern chemistry. Many similar facts have been proven long before Daulton's time, and these very facts are the ones that led him to the formation of his theory. All atoms of the same element are identical. All atoms of different elements are not alike.
Atoms combine with each other to form compounds.
A given compound has a number of different types of atoms. The elements that make up these compounds always combined in definite proportions by mass. O. This is known as the Law of Definite Proportions. Dalton's original theory was changed with the discovery of isotopes. An isotope contains the same number of protons but has a different number of neutrons.