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John Dalton
Dalton's theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights. He stated his theory in a lecture to the Royal Institution in 1803. The theory proposed a number of basic ideas:
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 -
Amedeo Avogadro
Amadeo Avogadro (1776-1856) proposed what is now known as Avogadro's Hypothesis in 1811. The hypothesis states that at the same temperature and pressure, equal volumes of gases contain the same number of molecules or atoms. When this is combined with Gay-Lussac's Law of Combining Volumes, the only possible formulas for hydrogen, oxygen and water are H2, O2 and H2O, respectively. The solution to the atomic weight problem was at hand in 1811. -
JJ Thomson
Thomson discovered the electron in the year 1897. His work put forward a new theory, that atom was made up of small particles.Thus he discovered the electrons. He proved his theory using the cathode ray tube.
Scientists had already done many experiments to find the structure of the atom. They passed an electric current through a vacuum tube. They saw a light glowing inside the tube. It travelled in a straight line. They could not explain the phenomenon. -
Robert Millikin
Robert Millikan determined the unit charge of the electron in 1909 with his oil drop experiment at the University of Chicago. An atomizer sprayed a fine mist of oil droplets into the chamber. Some of these tiny droplets fell through a hole in the upper floor. Millikan first let them fall until they reached terminal velocity. Using the microscope he measured their terminal velocity, and by use of a formula, calculated the mass of each oil drop. -
Ernest Rutherford
His famous gold foil experiment. This experiment involved the firing of radioactive particles through minutely thin metal foils (notably gold) and detecting them using screens coated with zinc sulfide (a scintillator). Rutherford found that although the vast majority of particles passed straight through the foil approximately 1 in 8000 were deflected leading him to his theory that most of the atom was made up of 'empty space'.
