History of the atom
In ancient Greece, most people thought that Sub-atomic particles and anything made from them, such as atoms and molecules, are matter. Energy and forces are not 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.
The plum pudding model
After discovering the electron in 1897, the English born physicist, J J Thomson, proposed the plum pudding model of the All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. in 1904.
To explain the two types of Electric charge that accumulates on an insulated object, for example because of friction. 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.