The development of the scientific model of the atom. Atomic Theory The development of the scientific model of the atom.

Earliest Atomic Theory 400 B.C. – Democritus, a Greek philosopher, developed the first atomic theory. He believed that matter was made up of tiny particles called atoms. He also believed that matter could not be created, destroyed, or further divided. His theory was met with criticism from other influential philosophers such as Aristotle. His theory was eventually rejected because it was not supported by experimental evidence.

Democritus’ Model of the Atom

1808-Dalton, an English school teacher, proposed an atomic theory 1808-Dalton, an English school teacher, proposed an atomic theory. His theory stated: All matter is composed of extremely small particles called atoms. All atoms of a given element are identical. Atoms of a specific element are different from those of any other element. Atoms cannot be created, destroyed, or divided into smaller particles. Different atoms combine in simple whole-number ratios to form compounds. In a chemical reaction, atoms are separated, combined, or rearranged.

Dalton’s Model of the Atom

Dalton’s Symbols for the Elements

Dalton’s theory was supported by experimental evidence which led to the general acceptance of his theory. His theory has had to be revised, however, as additional information has been learned.

Late 1890’s- Thomson, an English physicist, discovered the existence of a small negatively charged particle (the electron) through his cathode ray tube experiment. Link to Cathode Ray tube experiment animation

1909- Millikan, an American physicist, determined the charge of an electron through his Oil Drop Experiment. The value he determined is still the accepted value used today. Link to Millikan Oil Drop Experiment Animation

As a result of these discoveries, Thomson proposed a model of the atom that became known as the Plum Pudding model (or Chocolate Chip Cookie model).

1911- Rutherford, a New Zealand chemist, conducted the Gold-Foil Experiment. Link to Rutherford Gold Foil Experiment Animation

He recorded the following observations during his experiment: Most of the alpha particles passed through the gold foil undeflected. A smaller percentage of the particles were slightly deflected. A very small number of particles were deflected straight back towards their source.

From these observations, he concluded the following: The atom is mostly empty space. There is a tiny, dense central core in the atom. He called this the nucleus. The nucleus is positively charged.

From this experiment he concluded that the Plum Pudding model was incorrect.

1919- Goldstein concluded that the nucleus contained positively charged particles called protons. This particle carries a charge equal but opposite to that of an electron.

1932- Chadwick, an English physicist, showed that the nucleus also contains another subatomic particle. This neutral particle was called the neutron. Its mass is nearly equal to that of a proton, but it carries no charge.

As a result of this knowledge, a new model of the atom was developed by Rutherford and Bohr, a Danish physicist. This model is sometimes called the Planetary model because it describes the atoms as being similar to the solar system. The sun represents the nucleus and the planets represent the electrons.

1913-Moseley, an English physicist, conducted experiments using an X-ray tube. Through his experiment he discovered that each element had a specific number of protons. This number became referred to as the atomic number.

Summary of the Subatomic Particles Symbol Location Relative Charge Relative Mass Proton p+ Nucleus 1+ 1 Neutron no Electron e- In the space surrounding the nucleus 1- 1/1840