Models of the atom over time

Early ideas of matter

In ancient Greece, most people thought that matter was made up of combinations of four elements: earth, air, fire and water.

There were a small number of Greeks who had a different idea. They believed that if there was a piece of wood for example, it could be cut into smaller and smaller pieces until it ended up as a piece of wood that was so small it couldn’t be cut anymore.

The ancient Greek philosopher Demokritos (460-370 BCE) thought that matter was made up of millions of tiny, uncuttable pieces of that same matter. In fact, the word atom comes from the word ‘atomos’, which means uncuttable.

Plum pudding model of the atom, one big positive proton, with little electrons on top, like a cookie or a plum pudding.

After discovering the electron in 1897, J J Thomson, proposed that the atom looked like a plum pudding. To explain the two types of static electricity, he suggested that the atom consisted of positive ‘dough’ with a lot of negative electrons stuck in it. This was consistent with the evidence available at the time:

  • solids cannot be squashed, therefore the atoms which make them up must be solid throughout
  • rubbing two solids together often results in static charge so there must be something (electrons) on the outsides of atoms which can be transferred as atoms collide

Rutherford and the nucleus

In 1905, Ernest Rutherford did an experiment to test the plum pudding model. His two students, Hans Geiger and Ernest Marsden, directed a beam of alpha particles at a very thin gold leaf suspended in a vacuum.

Alpha particles are a form of nuclear radiation with a large positive charge.

The vacuum is important because any deflection of the alpha particles would only be because of collisions with the gold foil and not due to deflections off anything else.

Alpha radiation beamed to a thin piece of gold foil which is in a scintillation screen.

It was thought that the alpha particles could pass straight through the thin foil, or possibly puncture it. The scientists were very surprised when other things happened:

  • most of the alpha particles did pass straight through the foil
  • a small number of alpha particles were deflected by large angles (> 40°) as they passed through the foil
  • a very small number of alpha particles came straight back off the foil

Rutherford considered these observations and he concluded:

  • The fact that most alpha particles went straight through the foil is evidence for the atom being mostly empty space.
  • A small number of alpha particles being deflected at large angles suggested that there is a concentration of positive charge in the atom. Like charges repel, so the positive alpha particles were being repelled by positive charges.
  • The very small number of alpha particles coming straight back suggested that the positive charge and mass are concentrated in a tiny volume in the atom (the nucleus). The tiny number doing this means the chance of being on that exact collision course was very small, and so the ’target‘ being aimed at had to be equally tiny.
Alpha raditaion beamed through gold nuclei, with most passing through unaffected, some deflected at small angle and very few rays being almost reflected back.

Rutherford had discovered the nuclear atom, a small, positively-charged nucleus surrounded by empty space and then a layer of electrons to form the outside of the atom.

The discovery of the make-up of the nucleus (protons and neutrons) came much later, and was not made by Rutherford. The nucleus was calculated to be about 1/10,000th the size of the atom.