Atoms and Molecules

August 19, 2021


Photographer: Thomas McGuire

Summary Author: Thomas McGuire

The word atom originated thousands of years ago as a philosophical idea that there might be a smallest division to every substance. It was a concept with neither evidence nor application. But, in the early 1800s John Dalton observed that elemental substances combine in consistent numerical ratios, which provided a scientific basis for the idea of atoms. Still, this was indirect, non-observable evidence of atoms. Regardless, Dalton’s discovery led to many useful principles in chemistry.

There is, however, observational evidence of atoms in minerals. Each of the characteristic shapes of mineral crystals can be attributed to the spatial patterns of atoms and molecules. For example, quartz (SiO2) can form beautiful six-sided crystals because of the way atoms of silicon and oxygen join in hexagonal patterns, as shown above in the picture and illustration.

You can also see evidence of irreducible units if you deposit a small drop of a light, insoluble liquid such as oleic acid (the principal ingredient of olive oil) on a water surface. The oil will only spread out to a consistent limiting thickness before it breaks apart. This experiment has two implications. First, it is more evidence that there is some kind of irreducible unit of matter. (Subatomic particles are beyond the scope of these kinds of observations.) The second is that this is an indication of the diameter of one of those units even though oleic acid (C18H34O2) is a relatively large molecule. For comparison, oleic acid is 10-9 m compared to a hydrogen atom which is 10-11 m.