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500
Democritus
Democritus developed systems that made change possible by showing that it does not require that something should come to be from nothing. The shape and existence of all things are determined by void and atoms through the differences in shape, position and arrangement of the atoms and the proportion of void in the substance. They believed that there was an infinite number of atoms that were indivisible and impassable. -
Dalton
He experimented with mixed gases; on the pressure of steam and other vapors at different temperatures, both in a vacuum and in air; on evaporation; and on the thermal expansion of gases.
1) All matter is made of atoms. Atoms are indivisible and indestructible.
2) All atoms of a given element are identical in mass and properties
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms. -
Goldstien
Goldstein also used discharge tubes to investigate comets. An object, such as a small ball of glass or iron, placed in the path of cathode rays produces secondary emissions to the sides, flaring outwards in a manner reminiscent of a comet's tail. Goldstein concluded that in addition to the already-known cathode rays, later recognized as electrons moving from the negatively-charged cathode toward the positively-charged anode, there is another ray that travels in the opposite direction. -
Thomson
Thomson discovered this through his explorations on the properties of cathode rays. Thomson found that the rays could be deflected by an electric field (in addition to magnetic fields, which was already known). By comparing the deflection of a beam of cathode rays by electric and magnetic fields he was able to measure the particle's mass. This showed that cathode rays were matter, but he found that the particles were about 2000 times lighter than the mass of the lightest atom, hydrogen. He concl -
Rutherford
Discovered that uranium radiation is composed of positively charged 'alpha' particles and negatively charged 'beta' particles . First to prove radioactive decay of heavy elements and to perform a transmutation reaction. Discovered half-life of radioactive elements. Established that the nucleus was small, dense, and positively charged. Assumed that electrons were outside the nucleus. -
Schrodinger
Erwin Schrodinger added the final piece to the puzzle of electron arrangement around the nuclei of atoms. He suggested that electrons behave in a wave-like manner rather than just as particles and that their exact location within an orbit could not be precisely calculated. Schrodinger's view of the atom can be seen as "layers within layers" in terms of the electron shells. -
Chadwick
Chadwick made a fundamental discovery in the domain of nuclear science: he proved the existence of neutrons - elementary particles devoid of any electrical charge. Chadwick in this way prepared the way towards the fission of uranium 235 and towards the creation of the atomic bomb. -
Bohr
In atomic physics, the Bohr model, devised by Niels Bohr, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus.
He pointed out that in nuclear processes the smallness of the region in which interactions take place, as well as the strength of these interactions, justify the transition processes to be described more in a classical way than in the case of atoms.A liquid drop would give a very good picture of the nucleus.
