Democritus (460 BC-350BC) was a Greek philosopher who thought that

all matter was made of tiny indivisible particles he called atomos.

John Dalton, in the early 1800’s, gave us his version of the atomic

theory.

J.J. Thomson, a physicist, did a series of experiments with cathode ray tubes that proved the existence

of the electron.

Thomson continued to experiment and postulated that the atom

looked like plum pudding (think chocolate chip cookie) with protons and electrons in it.

Ernest Rutherford was a student of Thomson and wanted to learn

more. He conducted the gold foil experiment and found the

existence of the dense, positively charged nucleus.

Niels Bohr worked with Rutherford and went one step further. He said

the electrons were located in energy levels and gave us the

planetary model.

We progressed beyond the Bohr planetary model to today’s model

– one in which electrons are at specific energy levels but are

known to always be in motion and not orbiting the nucleus.