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Democritus
Abdera, Greece. Democritus though that atoms were the tiny indestructible units of matter. We now know that atoms were made up of solid parts and empty space. The atoms differed in shape and their contours showed an infinite variety and could be oriented in any direction and arranged in any order, so therefore, atoms could enter into countless combinations.
Sources: 1 2 -
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Dalton
Eaglesfield, Cumberland, England. Dalton’s theory stated that elements consisted of tiny particles called atoms. He said that the reason elements differed from one another was that atoms of each element were deferent from one another. Dalton’s theory became the theoretical foundation in chemistry. He also proposed the Law of Simple Multiple Proportions.
Source: http://dl.clackamas.edu/ch104-04/dalton%27s.htm -
G.J Stoney
Oak Park, Irish Midlands. His most scientific work was the conception and calculation of the magnitude of the “atom electricity”. He proposed the term ‘electron’ to describe the fundamental unit of electrical charge. His contributions to electrical charge laid the foundations for the discovery of the particle by J.J Thomson.
Sources: http://en.wikipedia.org/wiki/George_Johnstone_Stoney -
J.J Thomson
Cheetham Hill, Manchester , UK. Thomson suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. He discovered the electron in a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube, an area being investigated by numerous scientists at the time.
Source: http://www.chemheritage.org/discover/chemistry-in-history/themes/atomic-and-nuclear-structure/thomson.aspx -
Planck
Kiel, Duchy of Holstein. Max Planck is considered to be the inventor of Quantum Theory. He discovered a new fundamental constant and it is being used to calculate the energy of a photon. He discovered the law of heat radiation, which is named Planck's law of black body radiation. This law became the basis of quantum theory, which emerged ten years later in cooperation with Albert Einstein and Niels Bohr.
Source: http://www.timelineindex.com/content/view/1203 -
Nagaoka
He developed an early incorrect ‘planetary model’ of an atom. This model was based around an analogy to the explanation of the stability of the Saturn rings. The model made two predictions:
• A very massive nucleus (in analogy to a very massive planet)
• Electrons revolving around the nucleus, bound by electrostatic forces (in analogy to the rings revolving around Saturn, bound by gravitational forces).
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Millikan
Morrison, Illinois, USA. His earliest major success was the accurate determination of the charge carried by an electron, using the elegant "falling-drop method"; he also proved that this quantity was a constant for all electrons (1910). Because he proved that the quantity was constant for all electrons, thus, he demonstrated the atomic structure of electricity.
Source: http://corrosion-doctors.org/Biographies/MillikanBio.htm -
E. Rutherford
Brightwater, New Zealand. He publishes his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons. This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space.
Source: http://www.rsc.org/chemsoc/timeline//pages/1911.html -
H.G.J Mosley
Weymouth, Dorset, United Kingdom. Henry Moseley published the results of his measurements of the wavelengths of the X-ray spectral lines of a number of elements. He developed the application of X-ray spectra to study atomic structure; Moseley's discoveries resulted in a more accurate positioning of elements in the Periodic Table by closer determination of atomic numbers.
Source: http://www.chemistry.co.nz/henry_moseley.htm -
Bohr
Copenhagen, Denmark. Niels Bohr applies quantum theory to Rutherford's atomic structure by assuming that electrons travel in stationary orbits defined by their angular momentum. This led to the calculation of possible energy levels for these orbits and the postulation that the emission of light occurs when an electron moves into a lower energy orbit.
Source: [http://www.rsc.org/chemsoc/timeline//pages/1913.html](Copenhagen, Denmark) -
James Chadwick
Cambridge, England. James Chadwick proved that the atomic nucleus contained a neutral particle which had been proposed more than a decade earlier by Ernest Rutherford. He was the first man to discover the neutron.
Source: http://www.suite101.com/content/the-discovery-of-the-neutron-a46060
