The problems with atoms
Previous page: Introduction to Reacting Quantities
Chemists have worked hard over time to develop a system of measuring accurately the quantities of substances required for carrying out chemical reactions.
But it was not easy. There were two main problems:
(a) chemical substances are composed of very small particles which cannot be counted or weighed individually;
(b) because atoms are so small, if you weighed out even one gram of a substance it will contain an enormous number of atoms.
Quantities and equations
So chemists had to come up with a method of accurately measuring small and large amounts of the substances they were working with, taking into account those problems. They also wanted a method for measuring quantities of atoms that would be easily useable in the laboratory, and would not require special equipment. It would also be extremely useful if we could link the method of measurement developed to our chemical equations.
Note: Chemical equations tell us a lot already.
(a) They tell us the species involved in the reactions, for example Zn, PbO, H+.
(b) The equation usually includes phases (solid, liquid or gas).
(c) The equation shows the ratio of the species reacting.
So it would be handy if we could use the equation to tell us still more things such as the number of atoms reacting.
The quantity most easy to measure and work with in the laboratory is MASS. So it would certainly be useful if we could link together the mass of the reactants, or the products, involved in a reaction to the actual number of atoms reacting as indicated by the equation.
Developing an atomic mass scale
When we measure quantities, one of the aspects we almost always take for granted is the scale we use. Most of the scales we use in everyday life are based on real standards that we can relate back to. For example, the Celsius temperature scale is based around the melting and boiling points of water. Zero degrees is determined by the melting point of water, one hundred degrees by the boiling point of water.
When measuring atoms, we cannot use such scales as the basis for our measurement because, as was mentioned above, the individual atoms are so small that the numbers involved in a scale based on our everyday units of mass would be very small, and to complicate things further, atoms of the same element are not all the same mass (see Isotopes). So we have to use what is called a relative scale. A relative scale is based on comparing quantities so that we can gain a relative idea of size. For example, a dozen apples will weigh more than a dozen Smarties, so it is reasonably safe to say that each individual apple weighs more than each individual Smartie.
The Smarties Experiment is useful in helping you arrive at an understanding of how the relative atomic mass scale was derived.
The relative atomic mass scale used now actually gives masses relative to a specific atom of carbon which was assigned the value of 12.0000 units for entirely practical purposes. That is, if we had carried out an experiment based on the reaction:
C(s) + ZnO (s) → Zn (s) + CO (g)
then the ratio of the masses of carbon used in the experiment and zinc produced would be the same as the ratio of their atomic masses. We could thus assign zinc an atomic mass of 65.4 on a scale based on the experimental result that the mass of zinc metal we ended up with was 65.4/12 times the mass of carbon we started with.