|Event Date:||Event Title:||Event Description:|
|Democritus||Date: 460-370 BC
Idea/experiment: Continued the work of his mentor, Leucippus. He formulated an atomic theory similar to modern science’s understanding of the atom.
Contibution to the understanding of the atom: His theory suggested that atoms can’t be destroyed and exist in a void. Atoms only differ in shape, position and arrangement
|Isaac Newton||Date: 1704
Idea/experiment: Newton formulated a theory of light, the three laws of motion and suggested a mechanical universe with small, solid masses in motion.
Contribution to our understanding of the atom: Newton suggested that atoms are held together with attractions, otherwise known as forces. He also stated that "matter is formed of solid, massy impenetrable particles”.
|John Dalton||Date: 1803
Idea/experiment: He proposed an atomic theory.
Contribution to our understanding of the atom: Dalton stated (1) all matter was composed of small indivisible particles termed atoms, (2) atoms of a given element possess unique characteristics and weight, and (3) three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules).
|George Johnstone Stoney||Date: 1894
Idea/experiment: Introduced the term electron and estimated the charge carried by a single hydrogen atom.
How this contributed to our understanding of the atom: Stoney introduced the term electron as the “fundamental unit quantity of electricity”.
|Joseph John Thomson||Date: 1897
Idea/experiment: The discovery of the electron and of isotopes, and the invention of the mass spectrometer.
Contibution to our understanding of the atom: With the Nernst-Thomson rule he explains that it is difficult for charged ions to attract each other through insulating water molecules, so they dissociate.
|Max Planck||Date: 1900
Idea/experiment: A scientific philosopher Planck's Principle was that "A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die and a new generation grows up that is familiar with it."
Contribution to our understanding of the atom: Planck, a German physicist, is considered the founder of the quantum theory.
|Hantaro Nagaoka||Date: 1903
Idea/experiment: Nagaoka’s most notable work included the Saturnian model and his work on spectroscopy
Contribution to our understanding of the atom: Nagaoka created an early, incorrect model of an atom using an analogy based on Saturn’s rings. He also worked with British physicist C. G. Knott on spectroscopy.
|Robert Andrews Millikan||Date: 1909
Idea/experiment: Millikan is famous for his measurement of the charge of an electron and his work on the photoelectric effect.
Contribution to our understanding of the atom: Using an oil drop experiment Millikan measured the charge of an electron.
|Ernest Rutherford||Date: 1911
Location: Britain/ New Zealand
Idea/experiment: Rutherford was a chemist and a physicist who was also known as the father of nuclear physics
Contribution to our understanding of the atom: Rutherford discovered the concept of the radioactive half life. He also differentiated and named alpha and beta radiation. Rutherford is credited for performing the first splitting of an atom.
|Henry Gwyn-Jeffreys Moseley||Date: 1914
Idea/experiment: Studying under Rutherford, Moseley developed the application of X-ray spectra to study atomic structure.
Contibution to our understanding of the atom: By measuring the wavelengths of the x-rays given off by certain metals, Moseley was able to determine the number of positive charges in the nucleus of an atom. This was the first concept of the atomic number.
|Neils Bohr||Date: 1922
Idea/experiment: Fundamental contributions to the understanding of atomic structure and quantum mechanics.
Contribution to our understanding of the atom: By expanding on Rutherford’s earlier theory Bohr published his theory which suggested that electrons travel in specific orbits around the nucleus, that outer orbits can hold more electrons than the inner orbits and that these orbits determine the chemical properties of the atom.
|James Chadwick||Date: 1932
Idea/experiment: Famous for his discovery of the neutron.
Contriution to our understanding of the atom: Chadwick discovered the neutron, named because of the lack of electric charge, which was previously an unknown particle in the nucleus of an atom.