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101
Democritus
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. Of the mass of atoms -
John Dalton
All elements are composed of atoms. All atoms of the same elements have the same mass, and the atoms of different elements have different masses. Compounds contain atoms of one or more elements. -
J.J. Thomson
Cathode rays were produced in the side tube on the left of the apparatus and passed through the anode into the main bell-jar, where they were deflected by a magnet. Thomson detected their path by the fluorescence on a squared screen in the jar. He found that whatever the material of the anode and the gas in the jar, the deflection of the rays was the same. He concluded that the rays were composed of very light, negatively charged particles which were a universal building block of atoms. -
Hantaro Nagaoka
Nagaoka rejected Thomson's model on the ground that opposite charges are impenetrable. He proposed an alternative model in which a positively charged center is surrounded by a number of revolving electrons. Nagaoka's 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 a around the nucleus, bound by electrostatic forces -
Ernest Rutherford
He aimed a narrow beam at the gold. The screen around the gold was made of material that produced a flash of light when struck by the fast-moving alpha particles. He figured out that something was deflecting the light at certain points. This gave him the theory of the nucleus. -
Neils Bohr
His model focused on electrons. The electrons move with constant speed around the nucleus. If an atom gains or loses energy, the energy of the electrons change. Atoms have many energy levels. An electron in an atom can move from one energy level to another when a atom gains or loses energy. -
Werner Heisenberg & Erwin Schrodinger
The electron cloud model is an atom model wherein electrons are no longer depicted as particles moving around the nucleus in a fixed orbit. Instead, as a quantum mechanically-influenced model, we shouldn’t know exactly where they are, and hence describe their probable location around the nucleus only as an arbitrary. -
James Chadwick
In 1932 Chadwick discovered a previously unknown particle in the atomic nucleus. It come to be known as the neutron because of its lack of electric charge. Unlike the positively-charged alpha particles, which are repelled by the electrical forces present in the nuclei of other atoms, neutrons do not need to overcome any Coulomb barrier and can therefore penetrate and enter the nuclei of even the heaviest elements such as uranium-235 and plutonium.
