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.
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:
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.
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.
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:
Rutherford considered these observations and he concluded:
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.