The History of the Atom

Everyone today has heard of the word "atom" and has some idea what it is, but it is only really within the last 200 years that we have manged to figure this out. And there are still secrets left to uncover.

Ancient Greece

The first recorded mention of the word atom is from Ancient Greece*. Several philosophers of the period thought long and hard about what matter actually is. We can see matter everywhere: the chair, the table, the computer, water, even the air, but what is it? What do all of these things have something in common?

To attempt to answer these questions, a philosopher named Democritus is credited with the following thought experiment:

We have an incredibly sharp and precise knife. We then get something like an iron bar. We could cut the bar in half. Then we could take that piece and cut that in half. Then cut this piece in half again. And again. And again. And again, many times. Eventually we would get to a piece so small that we cannot cut any further. We would not have the smallest piece of matter. This smallest piece he called ἄτομος. The Greek word read as 'Atomos' meaning indivisible.

Democritus Atom

Although the word started off as "atomos", over the course of time the word changed into the word "atom" which we use today. Democritus went further with his theory. He reasoned that atoms from each substance would be different and the atom would have the same properties as the substance it came from. So an atom of iron would be hard and join together with hooks. A water atom would be slimy. A feather atom would be light and wispy. This may seem a little silly to us, but it would take 2 millennia before any more progress was made to overturn this line of thinking.

Enter the 1600s

Scientific discovery in Europe began a huge leap forward in the 1600s, the beginning of the scientific revolution. Many scientists were now making pioneering discoveries based on actual experiments, rather than just thought. For example :

  • In the 1600s Rene Descartes created the Cartesian coordinate system used in maths.
  • Robert Boyle formulated his law of ideal gas in 1662.
  • Robert Hooke discovered the cell in 1665.
  • Sir Isaac Newton discovered gravity in 1687.
  • Carl Linnaeus developed the system for classification of life, 1735.
  • Benjamin Franklin found that lightning is made of electrical energy in about 1751.
  • Thomas Bayes formulated Bayes Theorem which paved the way for Bayesian probability (posthumously released) in 1763.
  • Edward Jenner paved the way for the eradication of smallpox and control of other infectious diseases with his development of vaccines in 1796.

Chemistry also saw major new discoveries.

  • Anton Lavoisier discovered the law of the conservation of mass in 1789. Lavoisier found that when an experiment is performed in a closed system (where the substances cannot escape) the amount of mass does not change.
  • Another chemist around this time, Joseph Proust (although others made contributions), in 1779 formulated the law of definite proportions. This law states that different samples of the same compound will always contain the same proportion, by mass, of the elements that make it up.
  • At the beginning of the 1800s a chemist named John Dalton brought together some of these ideas and formulated a more modern idea of the atom.

John Dalton's Atom

John Dalton used some of the evidence from experiments that he and others had performed to update the theory of the atom. Importantly, he realised that there is a distinction between elements and compounds. As you saw in the topic of matter, every substance is made of either a single element, a compound, or a complex mixture of the two. Dalton said that elements are made of individual atoms, and that atoms of a particular element are all identical in size, mass, and chemical properties whilst atoms of different elements have slight different characteristics. This may seem similar to what Democritus had said, but it is different because Democritus thought that each substance was made of individual atoms of the same type and he had no concept about the relative sizes. Dalton also realised that compounds are made of atoms of more than one type of element, and that each compound is composed of these atoms in a specific ratio i.e. every unit of water is composed of exactly two atoms of hydrogen and one atom of oxygen. And lastly, when a reaction occurs the atoms are simply swapped around, removed or added to form new compounds. The atoms themselves cannot be destroyed or created. These points can be summed up as follows:

  1. Elements are composed of very tiny particles called atoms.
  2. Atoms of each element are identical to one another in size, mass and chemical properties.
  3. Compounds are formed from the combination of atoms from more than one element.
  4. The ratio of the atoms of the elements in a particular compound is always the same.
  5. Atoms cannot be created or destroyed.

We are now at the point where we have a more familiar model for an atom, next we will look at the key discoveries that lead to the discovery of some of the secrets that the atom holds.


*It was Democritus that is credited with this idea since it is the first record we have found, but it is likely that it was discovered before this time, and subsequently lost to history.