Near the end of the1700s, two laws about chemical reactions emerged without referring to the belief of an atomic theory. The first was the law of conservation of mass, come up by Antoine Lavoisier in 1789, which states that the total mass in a chemical reaction remains constant. The second was the law of definite proportions. First proven by the chemist Joseph Louis Proust in 1799, this law states that if a compound is broken down into its constituent elements, then the masses of the constituents will always have the same proportions, regardless of the quantity or source of the original substance
Dalton also believed atomic theory could explain why water absorbed different gases in different proportions e.g., he found that water absorbed carbon dioxide far better than it absorbed nitrogen. Dalton hypothesized this was due to the differences in mass and complexity of the gases' respective particles. Carbon dioxide molecules are heavier and larger than nitrogen molecules..
The flaw in Dalton's theory was corrected in principle in 1811. Avogadro had proposed that equal volumes of any two gases, at equal temperature and pressure, contain equal numbers of molecules. Avogadro's law allowed him to deduce the diatomic nature of numerous gases by studying the volumes at which they reacted. For instance, since two litres of hydrogen will react with just one litre of oxygen to produce two litres of water vapour, it meant a single oxygen molecule splits in two in order to form two particles of water. Thus, Avogadro was able to offer more accurate estimates of the atomic mass of oxygen and various other elements, and made a clear distinction between molecules and atoms.
In 1803 Dalton orally presented his first list of relative atomic weights for a number of substances. This paper was published in 1805, but he did not discuss their exactly how he obtained these figures. The method was first revealed in 1807 by his acquaintance Thomas Thomson, in the third edition of...