Notes

Around the year 430 b.c., a Greek philosopher named Democritus proposed the idea that matter is formed of small pieces that could not be cut into smaller parts. He used the word atomos, which means “uncuttable,” for these smallest possible pieces.

Using evidence from many experiments, John Dalton, an English chemist, began to propose his atomic theory and model for atoms.

Through a series of experiments around the start of the twentieth century, scientists discovered that atoms are made of even smaller parts. In 1897, another British scientist, J. J. Thomson, found that atoms contain negatively charged particles. Yet, scientists knew that atoms themselves had no electrical charge. So, Thomson reasoned, atoms must also contain some sort of positive charge.

In 1911, one of Thomson’s students, Ernest Rutherford, found evidence that countered Thomson’s model.

In 1913, Niels Bohr, a Danish scientist and a student of both Thomson and Rutherford, revised the atomic model again. Bohr showed that electrons could have only specific amounts of energy, leading them to move in certain orbits. The series of orbits in Bohr’s model resemble planets orbiting the sun or the layers of an onion.