0.1 Atomic molecular theory  (Page 5/6)

We will now look in great detail at a few compounds formed just from nitrogen and oxygen, simply called nitrogen oxides. Since we don’t know anything about these compounds, for now we’ll just call them Oxide A, Oxide B, and Oxide C. These three compounds are very different from one another. Two of these are quite toxic, but one is used as an anesthetic, particularly by dentists. Two of these are colorless, but one is a dark brown color. Let’s look at the mass ratios for these three compounds in [link] .

At first glance, there is nothing special about these numbers. There are no obvious patterns or relationships amongst the masses or mass ratios. But let’s look at this data in the same way as we did in [link] by finding the mass of oxygen that combines with 1.00 g of nitrogen. This is in [link] .

You might have to look at these data very hard to see it, but there is a pattern that is obvious once you see it. In the column for the Mass of Oxygen, the three values listed have a simple relationship: each one is a multiple of 0.57. We can see this most clearly if we divide each of the masses for the three oxides by 0.57. This shows us that the ratio 2.28 : 1.14 : 0.57 is equal to the ratio 4 : 2 : 1.

What does this tell us? It means that if we have a fixed mass of nitrogen, the mass of oxygen which will combine with it cannot be simply any amount. In fact, the opposite is true. There are a few specific masses of oxygen which will combine with the fixed nitrogen, and those specific masses are integer multiples of a fixed unit of mass. It is particularly interesting that the masses of oxygen are in integer ratios. Integers are a special set of numbers used for one primary purpose, which is to count objects. In this case, the “object” must be a fixed unit of mass of oxygen.

The data in [link] tell us that when we have a fixed amount of nitrogen, it can be combined only with some integer number of a fixed unit of mass of oxygen. Why would there be a fixed unit of mass of oxygen? The simplest and best explanation is that oxygen exists as fixed units of mass, or particles, and we call these particles “atoms” of oxygen. Thus, the data in [link] lead us to a conclusion that the element oxygen is composed of individual atoms with identical mass. We have shown that matter is made up of particles and that elements consist of identical particles or atoms.

We can see these simple integer ratios in other compounds, as well. Let’s look back at [link] , which shows compounds of carbon and hydrogen. The ratios of the masses don’t appear to be interesting until we do the same type of analysis that we did on the nitrogen oxides. Let’s fix the mass of hydrogen in each of these compounds and find out the masses of carbon. The results are in [link] .