History of Atomic Theory



460 BC - 370 BC

Democritus, had a different theory. He believed that elements could be broken down into a basic level, which couldn't be broken down any further. He called these atoms, after the Greek word, ATOMOS, which means indivisible


384 bc - 322 bc

Aristotle was the first person to suggest that the world was made up of elements, which he believed to be Earth, Wind, Fire and Water. His theory was that elements are infinite and can be continuously broken down.

John Dalton

1766 - 1844

Dalton was the first person to prove which one, either Democritus or Aristotle, was right. Dalton hypothesised that atoms DO exist, disproving Aristotle. He then continued his research and developed the first part of the atomic theory. He stated that:
1. Matter is made up of tiny particles (atoms) which cannot be divided into smaller pieces or destroyed
2. Atoms of the same element, no matter where, will be exactally the same.
3. Atoms of different elements can combine to form compound elements.

J.J. Thomson

1856 - 1940

Thomson's greatest discovery was discovering the electron. He discovered the electron by placing a magnet inside a CATHODE RAY TUBE. The material which was produced was negatively charged. He eventually discovered that the mass of the negatively charged particles was 2000 times lighter than the mass of a hydrogen atom.

Hantaro Nagaoka

1865 - 1950

Nagaoka was the first person to SUGGEST that there was something in the middle of an atom, and there was charged... well, stuff, which revolved around the outside. He based his model on the planet Saturn, thus the name of the model, The Saturnian Model, developed in1904.

Robert Millikan

1868 - 1953

Robert Millikan discovered and PROVES the charge of an electron. HE does this by conducting his famous oil drop experiment. The experiment works by dropping oil drops through a narrow slit in the top of the main chamber. Then a positive charge is sent through the chamber to intercept the drop. When the voltage of the positive charge reaches a high enough voltage, it repels the drop, suspending it in the air or even making it rise.

Ernest Rutherford

1871 - 1937

Rutherford was the first person to PROVE that the atom has a POSITIVE, solid centre and NEGATIVE particles around the outside, and that majority of the atom was empty space. This was proved with his famous GOLD FOIL experiment, where α (alpha) particles were fired at a sheet of gold foil. He noted that approximately 1 in 8000 deflected the sheet, meaning that they hit something that was the same charge.

Niels Bohr

1885 - 1962

Bohr, after researching and further expanding the atomic design created by Rutherford (Saturnian model), he proposed a new design, called the Bohr model. This was the first design which answered majority of the questions which scientists posed at the time. It showed the electrons in the electron shells, the amount of protons and neutrons, and also how light is emitted from the electrons.

Henry Mosley

1887 - 1915

Mosley was a young and clever scientist who redesigned the periodic table. He discovered that the periodic table should not be ordered by their mass number, such as Mendeleev, the creator of the periodic table had arranged it, and instead by their atomic number. Problems like iodine (Atomic #53) coming before tellurium (Atomic #54) because it was lighter. However, when the periodic table is listed by atomic number, iodine comes after tellurium. However, Mosley was K.I.A at Gallipoli before he could continue his scientific career.

James Chadwick

1891 - 1974

James Chadwick was a student of Ernest Rutherford, who over saw his Ph.D. and made him assistant director of the lab in Cambridge university. His own research was on radioactivity. Then in 1919, Rutherford hypothesised the existence of a neutral particle within the nucleus, however, Chadwick and the other scientists couldn't find it.

Later, Chadwick used the same technique that Fredrick and Irene Joliot-Curie used to track radiation, except he was looking for a particle without a charge and with a similar mass of a proton. He found the neutron and received the Nobel Prize for his published piece, The Existence of a Neutron in 1932.