Timeline of Atomic Theory

Democritus

460

Wikipedia-DemocritusTimeline IndexDate: 460-370 BC
Location: Greece
Idea/Experiment: Continued the work of his mentor, Leucippus
Formulated an atomic theory similar to modern science’s understanding of the atom
How this contributed to our 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

Jan 1, 1704

Timeline on Atomic StructureScience World Biography: NewtonWikipedia-Isaac NewtonDate: 1704
Location: England
Idea/Experiment: Newton formulated a theory of light, the three laws of motion and suggested a mechanical universe with small, solid masses in motion.
How this contributed 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

Jan 1, 1803

Science World Biography: DaltonWikipedia- John DaltonDate: 1803
Location: England
Idea/Experiment: He proposed an atomic theory
How this contributed 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

Jan 1, 1894

Science World Biography: StoneyWikipedia- G. J. StoneyDate: 1894
Location: Ireland
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

Jan 1, 1897

Wikipedia- J. J. ThomsonScience World Biography: ThomsonDate: 1897
Location: England
Idea/Experiment: The discovery of the electron and of isotopes, and the invention of the mass spectrometer.
How this contributed 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

Jan 1, 1900

Wikipedia- Max PlanckScience World BiographyDate: 1900
Location: Germany
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."
How this contributed to our understanding of the atom: Planck, a German physicist, is considered the founder of the quantum theory.

Hantaro Nagaoka

Jan 1, 1903

Reference Answers Hantaro NagaokaWikipedia- Hantaro NagaokaDate: 1903
Location: Japan
Idea/Experiment: Nagaoka’s most notable work included the Saturnian model and his work on spectroscopy.
How this contributed 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

Jan 1, 1909

Science World Biography: Robert MillikanWikipedia- Robert Andrews MilllikanDate: 1909
Location: America
Idea/Experiment: Millikan is famous for his measurement of the charge of an electron and his work on the photoelectric effect.
How this contributed to our understanding of the atom: Using an oil drop experiment Millikan measured the charge of an electron.

Ernest Rutherford

Jan 1, 1911

Science World Biography: RutherfordWikipedia- Ernest RutherfordDate: 1911
Location: Britain/New Zealand
Idea/Experiment: Rutherford was a chemist and physicist who was known to be the father of nuclear physics.
How this contributed 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 Moseley

Jan 1, 1914

Henry MoseleyWikipedia- Henry MoseleyScience World Biography: Henry MoseleyDate: 1914
Location: England
Idea/Experiment: Studying under Ernest Rutherford, Moseley developed the application of X-ray spectra to study atomic structure.
How this contributed 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

Jan 1, 1922

Crystalinks: Neils BohrLucid Cafe Library: Neils BohrDate: 1922
Location: Denmark
Idea/Experiment: Fundamental contributions to the understanding of atomic strucutre and quantum mechanics.
How this contributed 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 elctrons than the inner orbits and that these orbits determine the chemical properties of the atom.

James Chadwick

Jan 1, 1932

Nobelprize: James ChadwickWikipedia- James ChadwickDate: 1932
Location: England
Idea/Experiment: Famous for his discovery of the neutron/
How this contributed to our understanding of the atom: Chadwick discovered the neutron, named because of its lack of electrical charge, which was previously an unknown particle in the nucleus of an atom.