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400
Democritus
Democritus' major contribution is proposing the atomic theory of matter. He was born in Abdera, Greece in 460 BC. He published over 70 books, enjoyed travelling and visited many places. He lived for 90 years and died in 370 BC. -
400
Aristotle
Aristole's major contribution was stating that all substances were made of four elements.
1) earth
2) water
3) air
4) fire
He was born in Stagiro, Greece in 384 BC. He thought, along with everyone else, that Earth was in the center. A third of his writings were saved. The rest was either lost or destroyed. He developed the scientific method. He lived for 62 years and died in 322 BC. -
John Dalton
John Dalton's major contribution was transforming Democritus' ideas on atoms into a modern atomic model. His theory was:
1) All matter is made of microscopic particles called atoms.
2) The atoms of the same element are identical, those of different elements are different.
3) Atoms of different elements combine in small, whole number ratios to form compounds.
4) Chemical reactions involve the combinations of atoms. No new atoms are created. ( Law of simple multiple proportions). -
J.J. Thomson 1897
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Robert Millikan 1909
Robert Millikan's major contribution was determining the mass and charge of the electron. He conducted an oil drop experiment by measuring the effect of an electric field on very small oil droplets. From the mass of the drop and the charge on the plates, he calculated the charge on an electron. He was born in Morrison, USA on the 22nd of March, 1886. He lived for 67 years and died on 19th Decenber, 1953. -
Ernest Rutherford 1912
Ernest Rutherford discovered the nucleus. He did a Gold Foil Experiment. Ernest Rutherford made 2 major conclusions from his Gold Foil Experiment. 1.) The nucleus is made up of mostly +charge and mass. 2.) The atom has a positive piece at the center called a nucleus. -
Neils Bohr 1913
Neils Bohr's major contribution was proposing that an electron is found only in specific circular paths, or orbit, around the nucleus. Bohr's model was referred to as planetary model becauseelectrons move around the nucleus like planets around the sun. -
Sir James Chadwick
Sir James Chadwick's major contribution was confirming the existence of neutron in the atom. The modern current view was:
1) The atom is mostly empty space.
2) The atom consists of two regions.
3) Nuecleus in center and contains protons and neutrons.
4) The electron cloud is the region where electrons in areas of high probability called atomic orbital.