0.1 The atomic molecular theory  (Page 2/4)

However, if we do decide to jump to conclusions and assume the existence of atoms without further evidence (as did the leading chemists of the seventeenth and eighteenth centuries), it does not lead us anywhere. What happens to iron when, after prolonged heating in air, it converts to iron rust? Why is it that the resultant combination of iron and air does not maintain the properties of either, as we would expect if the atoms of each are mixed together? An atomic view of nature would not yet provide any understanding of how the air and the iron have interacted or combined to form the new compound, and we can't make any predictions about how much iron will produce how much iron rust. There is no basis for making any statements about the properties of these atoms. We need further observations.

Observation 1: mass relationships during chemical reactions

The Law of Conservation of Mass, by itself alone,does not require an atomic view of the elements. Mass could be conserved even if matter were not atomic. Theimportance of the Law of Conservation of Mass is that it reveals that we can usefully measure the masses of theelements which are contained in a fixed mass of a compound. As an example, we can decompose coppercarbonate into its constituent elements, copper, oxygen, and carbon, weighing each and taking the ratios of thesemasses. The result is that every sample of copper carbonate is 51.5% copper, 38.8% oxygen, and 9.7%carbon. Stated differently, the masses of copper, oxygen, and carbon are in the ratio of 5.3 : 4 : 1, for everymeasurement of every sample of copper carbonate. Similarly, lead sulfide is 86.7% lead and13.3% sulfur, so that the mass ratio for lead to sulfur in lead sulfide is always 6.5 : 1. Every sample of coppercarbonate and every sample of lead sulfide will produce these elemental proportions, regardless of how much material wedecompose or where the material came from. These results are examples of a general principle known as the Law of Definite Proportions .

Law of definite proportions

These data help justify an atomic view of matter. We can simply argue that, for example, leadsulfide is formed by taking one lead atom and combining it with one sulfur atom. If this were true, then we alsomust conclude that the ratio of the mass of a lead atom to that of a sulfur atom is the same as the 6.5 : 1 lead to sulfur mass ratio we found for the bulk leadsulfide. This atomic explanation looks like the definitive answer to the question of what it means to combinetwo elements to make a compound, and it should even permit prediction of what quantity of lead sulfide will be producedby a given amount of lead. For example, 6.5g of lead will produce exactly 7.5g of lead sulfide, 50g of lead willproduce 57.7g of lead sulfide, etc.

There is a problem, however. We can illustrate with three compounds formed from hydrogen, oxygen,and nitrogen. The three mass proportion measurements are given in the following table . First we examine nitric oxide, to find that the mass proportion is 8 : 7 oxygen to nitrogen. Ifthis is one nitrogen atom combined with one oxygen atom, we would expect that the mass of an oxygen atom is 8/7=1.14 timesthat of a nitrogen atom. Second we examine ammonia, which is a combination of nitrogen and hydrogen with the massproportion of 7 : 1.5 nitrogen to hydrogen. If this is one nitrogen combined with one hydrogen, we would expect thata nitrogen atom mass is 4.67 times that of a hydrogen atom mass. These two expectations predict a relationshipbetween the mass of an oxygen atom and the mass of a hydrogen atom. If the mass of an oxygen atom is 1.14 times the mass ofa nitrogen atom and if the mass of a nitrogen atom is 4.67 times the mass of a hydrogen atom, then we must conclude thatan oxygen atom has a mass which is 1.14×4.67 = 5.34 times that of a hydrogen atom.