0.1 The atomic molecular theory  (Page 3/4)

But there is a problem with this calculation. The third line of the following table shows that the compound formed from hydrogen and oxygen is water, which is found tohave mass proportion 8:1 oxygen to hydrogen. Our expectation should then be that an oxygen atom mass is 8.0times a hydrogen atom mass. Thus the three measurements inthe following table appear to lead to contradictory expectations of atomic mass ratios. How are we toreconcile these results?

One possibility is that we were mistaken in assuming that there are atoms of the elements which combine to form the different compounds. If so, then we would not be surprised to see variations in relative masses of materials which combine.

Another possibility is that we have erred in our reasoning. Looking back, we see that we have to assume how many atoms of each type are contained in each compound to find the relative masses of the atoms. In each of the above examples, we assumed the ratio of atoms to be 1:1 in each compound. If there are atoms of the elements, then this assumption must be wrong, since it gives relative atomic masses which differ from compound to compound. How could we find the correct atomic ratios? It would help if we knew the ratio of the atomic masses: for example, if we knew that the oxygen to hydrogen mass ratio were 8:1, then we could conclude that the atomic ratio in water would be 1 oxygen and 1 hydrogen. Our reasoning seems to circular: to know the atomic masses, we must know the formula of the compound (the numbers of atoms of each type), but to know the formula we must know the masses.

Which of these possibilities is correct? Without further observations, we cannot say for certain whether matter is composed of atoms or not.

Observation 2: multiple mass ratios

Significant insight into the above problem is found by studying differentcompounds formed from the same elements. For example, there are actually three oxides of nitrogen, that is,compounds composed only of nitrogen and oxygen. For now, we will call them oxide A, oxide B, and oxideC. Oxide A has oxygen to nitrogen mass ratio 2.28 : 1. Oxide B has oxygen to nitrogen mass ratio 1.14 : 1,and oxide C has oxygen to nitrogen mass ratio 0.57 : 1.

The fact that there are three mass ratios might seem to contradict theLaw of Definite Proportions, which on the surface seems to say that there should be just oneratio. However, each mass combination gives rise to a completely unique chemical compound with verydifferent chemical properties. For example, oxide A is very toxic, whereas oxide C is used as ananesthesia. It is also true that the mass ratio is not arbitrary or continuously variable: we cannot pickjust any combination of masses in combining oxygen and nitrogen, rather we must obey one of only three. Sothere is no contradiction: we simply need to be careful with the Law of DefiniteProportions to say that each unique compound has a definite mass ratio of combining elements.