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380
Democritus
The Greek philosopher Democritus proposed that matter was formed of small pieces called atoms which means indivisible in Greek. -
383
Aristotle
At the age of 49, Aristotle had started his own school called Lyceum. He had stated that all substances and made of 4 elements -
1) Fire
2) Earth
3) Air
4) Water. -
John Dalton
John Dalton had transformed Democritus' ideas on atoms into a theory. Dalton's Atomic theory-
1) All matter is made of tiny particles called atoms.
2) All atoms of a same element are alike, those of different elements are different.
3) Atoms of different elements combine in small, numerical ratios to form compuonds.
4) Chemical reactions involve the rearrangement of atoms, not a change int he atoms themselves. -
J.J. Thomson
J.J. Thomson had discovered the electron. He had conducted the cathode ray tube experiment. By adding an electric charge he found that the moving pieces were electrons. -
Robert Millikan
Robert Milikan's major contribution was that he determined the mass and the charge of the electron. He had conducted an oil drop experiment in which he measured the effect of an electric field on very small oil droplets. -
Ernest Rutherford
Ernest Rutherford had discovered the nucleas of an atom. Mr. Rutherford had conducted a gold foil experiment and made 2 major conclusions -
1) The nucelus is made of mostly Protons
2) The atom contains a hard surface peice at the center. -
Neils Bohr
Neils bohr had proposed that an electron is found only in specific circular paths around the nucleus.
1) Electrons do not randomly orbit the nucleus.
2) Electrons move in specific shells.
3) Atoms absorb or give off energy when electrons move form one level another. -
Sir James Chadwick
Sir James Chadwick's major contribution was that he confirmed the existence of neutrons in the nucleus. Sir James proved Rutherford's atomic theory and revolutionized the understanding of the atomic structure.
