-
370
B.C. Democritus
Democritus was an influential pre-Socratic philosopher who formulated an atomic theory for the cosmos. The theory of Democritus and Leucippus held that everything is composed of "atoms", which are physically, but not geometrically, indivisible; that between atoms lies empty space that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size. -
Period: 460 to
History of Atomic Theory & Atomic Model
History of Atomic Theory & Atomic Model from 460 B.C.E to modern model. -
Jan 7, 600
B.C. Aristotle
Aristotle’s theory made a great generalization off all matter being made of the four elements: fire, water, earth, and air. He also believed that there were four qualities to these elements: dryness, hotness, coldness, and moistness. Based on these beliefs fire would hold the characteristics of being dry and hot, water is wet and cold, air is hot and wet, while the earth is dry and cold. -
Dalton's Atomic Theory
All matter is composed of atoms
Atoms cannot be made or destroyed
All atoms of the same element are identical
Different elements have different types of atoms
Chemical reactions occur when atoms are rearranged
Compounds are formed from atoms of the constituent elements. -
J.J. Thomson's Atomic Model
In 1904 Thomson suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. His efforts to estimate the number of electrons in an atom from measurements of the scattering of light, X, beta, and gamma rays initiated the research trajectory along which his student Ernest Rutherford moved. Thomson's last important experimental program focused on determining the nature of positively charged particles. -
Robert A. Millikan's Atomic Theory
Millikan worked on an oil-drop experiment in which he measured the charge on a single electron. J.J. Thomson had already discovered the charge-to-mass ratio of the electron. However, the actual charge and mass values were unknown. Therefore, if one of these two values were to be discovered, the other could easily be calculated. Millikan and his then graduate student Harvey Fletcher used the oil-drop experiment to measure the charge of the electron. -
The Ruthreford Model
Rutherford directed the famous Geiger-Marsden experiment in 1909, which suggested, upon Rutherford's 1911 analysis, that the so-called "plum pudding model" of J. J. Thomson of the atom was incorrect. Rutherford's new model[1] for the atom, based on the experimental results, contained the new features of a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom and with this central volume also containing the bulk of the atomic mass of the atom. -
The Bohr Model
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. -
Modern Atomic Theory
French physicist Louis-Victor de Broglie (1892–1987) expanded on this theory by proposing that all matter, as well as radiation, behaves both as a particle and a wave. Until this time, scientists had viewed matter and energy as distinct phenomena that followed different laws.
