Atomic Structure Throughout the Ages

John Dalton

Mar 22, 1803

1803
Proposed an ‘atomic theory’ with spherical atoms based upon measurable properties of mass
The main points of Dalton’s atomic theory were:
Elements are made of extremely small particles called atoms
Atoms of different elements are different in size and mass.
Technology/method:
Analysis of various chemicals to determine mass of different atoms.
Classical analytical chemistry techniques including separations such as precipitation, extraction and distillation.

JJ Tompson

Mar 22, 1897

1897
Atoms contain negatively charged particles called electrons
Model suggested atoms are positively charged spheres with negatively charged electrons embedded in them
Like the fruit in a plum pudding
Technology/method:
Series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube

Ernest Rutherford

Mar 22, 1911

1911
Atoms consist of mostly empty space with dense nucleus containing positively charged protons in the centre and electrons orbiting the nucleus.
This model is very similar to today’s model, but with one flaw; it is proposed that the orbiting electrons would lose energy and spiral towards the nucleus.
Technology/method:
Gold foil experiment showing the tiny atom has a heavy nucleus
Designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world

Niels Bohr

Mar 22, 1913

1913
Electrons orbit the nucleus at different energy levels.
Only electrons with specific amounts of energy could exist at each level. His model proposed that electrons could move from one level to another by gaining or losing ‘packets’ of energy. Although Bohr’s model explained why electrons did not spiral in towards the nucleus, it did not explain all of the known properties of atoms.
Technology/method:
Discovered while conducting research for his doctoral thesis on on the electron theory of

James Chadwick

Mar 22, 1932

1932
The nucleus contains particles called neutrons as well as protons.
Neutrons have no electric charge and around the same mass as a proton.
Technology/method:
Scattering data to calculate the mass of this neutral particle

Murray Gell-Mann

Mar 22, 1961

1961
The magnetic properties of protons and neutrons require the existence of three new elementary particles, known as quarks.
Technology/method:
He drafted a blueprint for subatomic physics known as the Eightfold Way and in the process discovered the need for three new particles, which he named quarks.

George Zweig

Mar 22, 1964

1964
Both mesons and baryons are made up of three fundamental particles [quarks].
Technology/method:
Unsure