Atoms – basic ideas

The original idea of an atom came from asking a very simple question: if you take a substance and cut it into smaller and smaller pieces, is there a limit to how many times you can keep cutting it into smaller pieces?

When this question was originally asked, back in ancient Greece around 400 BC, nobody could prove the answer one way or the other so everything came down to a matter of belief. The correct answer was the one expressed by Democritus, who believed that you can’t keep cutting things into smaller and smaller pieces: eventually you reach a limit. The name given to the smallest piece of anything was “atomos”, meaning indivisible (cannot be made smaller) and that gave us the modern word “atom”.

The idea that an atom is the smallest part of anything needs one small correction: an atom is the smallest part of a pure substance, where all the particles in the substance are exactly the same.

Let’s think about pure water. We know that water is expressed chemically as “H-2-O” and this tells us that it contains two different types of particles; one of type “H” and the other of type “O”. These are, of course, particles of hydrogen and oxygen.

The smallest part of water is one particle of “H-2-O” but this is not an atom because it can be split into particles of hydrogen and particles of oxygen. (We can do this quite easily by passing electricity through water.) Once we have separated the hydrogen from the oxygen, we can start dividing them down until we get to the very last particles; one atom of hydrogen and one atom of oxygen.

The idea of an atom is really important, as is the idea of pure substances.

There are only 118 different pure substances, or elements, and all of them are listed in the Periodic Table. The arrangement of the Periodic Table is not random but neither does it have an obvious pattern, such as alphabetically or even in order of each element’s discovery. Originally, the Periodic Table was arranged by the mass of the elements but later it was refined so that the elements are listed by atomic number.

To understand atomic number we have to realise that Democritus wasn’t totally right in his ideas because it turns out that atoms can be cut after all. But when they are cut, they stop having the properties of their original substance – so Democritus was right for most practical purposes.

When we cut into atoms we find three new particles, called electrons, protons and neutrons. An electron has very little mass and carries a negative electrical charge; a proton has about 2000x the mass of an electron and carries a positive charge; a neutron has roughly the same mass as a proton but no electrical charge at all (it is neutral, hence its name).

As elements gain more protons they also gain more mass so it shouldn’t make too much difference whether mass or proton number is used to put the elements in order in the Periodic Table. Sadly, the differences may be small but they are important so instead of putting elements into the Periodic Table by mass, we actually arrange the elements in order of increasing numbers of protons, which is also increasing numbers of electrons.

It took thousands of years to move from the idea of an atom to being able to prove that atoms exist and that they have sub-atomic particles inside them. The history of models of the atom is discussed in a separate post (here) but from a chemical point of view, the only thing that matters is the arrangement of the electrons in an atom. For that information, you should read this post next.