<P> Near the end of the 18th century, two laws about chemical reactions emerged without referring to the notion of an atomic theory . The first was the law of conservation of mass, formulated by Antoine Lavoisier in 1789, which states that the total mass in a chemical reaction remains constant (that is, the reactants have the same mass as the products). The second was the law of definite proportions . First proven by the French 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 . </P> <P> John Dalton studied and expanded upon this previous work and developed the law of multiple proportions: if two elements can be combined to form a number of possible compounds, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers . For example: Proust had studied tin oxides and found that their masses were either 88.1% tin and 11.9% oxygen or 78.7% tin and 21.3% oxygen (these were tin (II) oxide and tin dioxide respectively). Dalton noted from these percentages that 100g of tin will combine either with 13.5 g or 27g of oxygen; 13.5 and 27 form a ratio of 1: 2 . Dalton found that an atomic theory of matter could elegantly explain this common pattern in chemistry . In the case of Proust's tin oxides, one tin atom will combine with either one or two oxygen atoms . </P> <P> Dalton believed atomic theory could explain why water absorbed different gases in different proportions - for example, 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 . Indeed, carbon dioxide molecules (CO) are heavier and larger than nitrogen molecules (N). </P> <P> Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures (chemical compounds). This marked the first truly scientific theory of the atom, since Dalton reached his conclusions by experimentation and examination of the results in an empirical fashion . </P>

Who are the 3 physicist developed a better model of the atom