According to the atomic theory, atoms are the units of chemical reactions. The formula HgBr2 indicates that each molecule of this substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. contains one mercury and two bromine atoms. Therefore, if we ask how much bromine is required to make a given quantity of mercury (II) bromide, the answer is two bromine atoms for each mercury atom or two bromine atoms per molecule. In other words, how much substance we have depends in a very important way on how many atoms or molecules are present.
So far, we've dealt with massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. ratios. Is there a way to change masses of atoms into numbers of atoms, so it is easy to see how much of one elementA substance containing only one kind of atom and that therefore cannot be broken down into component substances by chemical means. will react with another, just by looking at the number of atoms that are needed?
As we see below, there seems to be no fundamental connection between the number of atoms or molecules in the chemical equations, and typical measures of "how much":
|1 Hg (l)||+ 1 Br2 (l)||→ 1 HgBr2 (s)|
|1 atom||1 molecule||1 molecule|
|1.00 g||0.797 g||1.797 g|
|1.00 ml||3.47 ml||0.30 ml|
"How much?" in the above sense of the quantity of atoms or molecules present is not the same thing as how much in terms of volume or mass. It takes 3.47 cm3 Br2(l) to react with a 1-cm3 sample of Hg(l). That same 1 cm3 Hg(l) would weigh 13.59g, but only 10.83 g Br2(l) would be needed to react with it. In terms of volume, more bromine than mercury is needed, while in terms of mass, less bromine than mercury is required. In the atomic sense, however, there are exactly twice as many bromine atoms as mercury atoms and twice as much bromine as mercury.
Luckily, the International System of Measurements (IUPAC) has a measure of amount that reflects the number of atoms present, and it is called the mole.