John Dalton's first paper on the atomic theory appeared in 1803. It demonstrated the usefulness of assuming each element had a smallest unit. Dalton was studying the properties of gasses, particularly the ratios of volumes of gasses combining to form a chemical compound. Eg. Hydrogen + Oxygen = Water

Dalton’s results led to "the law of multiple proportions" (this law states that when 2 elements combine to form different compounds the weights of one of the elements which will unite with identical amounts of the second element.)

Dalton’s 2nd law, "the law of definite proportions" states that in any chemical compound the proportions by weight of the elements are always constant.

Dalton’s atomic laws were an improvement over the speculations of the Greek Atomists, since Dalton’s theory was capable of explaining and the results of actual experiments and of leading to predictions regarding the results of new experiments.

He said that:
1)All Atoms of any one element are exactly alike in all aspects; in particular, same weight.
2)Atoms of different elements have different properties; in particular, different weight.
3)A compound, as distinguished from an element is composed of a certain definite integral number of atoms of each of the elements present.
4)In the chemical reactions that may occur, none of the atoms are destroyed and no new ones are created.

Dalton’s atomic theory, based on these 4 postulates, was the simplest method of explaining the laws of definite proportions and multiple proportions.

Although Dalton’s laws are basic to the chemistry of compounds, they don’t actually define the size or the weight of the compound. For this, the total # of atoms in the given weight of elements or compounds must be known. Molecules of simple substances are too small to isolate and weight individually, so that indirect means must be used to determine molecular and atomic weights.