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Brief History of Atomic Theory

Brief History of Atomic Theory. 1 st atomic models. In 400 BC , the model looked like a solid indivisible ball. Atomos. Democritus’ theory: Matter could not be divided into smaller and smaller pieces forever He named the smallest piece of matter “atomos,” an indivisible particle.

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Brief History of Atomic Theory

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  1. Brief History of Atomic Theory

  2. 1st atomic models In 400 BC, the model looked like a solid indivisible ball

  3. Atomos • Democritus’ theory: Matter could not be divided into smaller and smaller pieces forever • He named the smallest piece of matter “atomos,” an indivisible particle

  4. This theory was ignored and forgotten for more than 2000 years!

  5. Why? Aristotle & Platothought earth,fire, air and water were the elements. Alchemy used this model for about 2000 years.

  6. Dalton’s Model In the early 1800s, the English chemist John Daltonperformed experiments that confirmed the idea of atoms.

  7. Dalton’s Atomic Theory • All elements are made of indivisible atoms • Atoms of the same element are identical, but differ from atoms of other elements • Different elements’ atoms can combine to form compounds. • Reactions occur when atoms are separated, joined, or rearranged. WHICH OF THESE ARE NO LONGER CONSIDERED TO BE TRUE ?

  8. Thomson’s Plum Pudding Model In 1897, J.J. Thomson discovered that atoms are made of even smaller particles.

  9. Thomson Model • Thomson used cathode ray tube • The ray consisted of negatively charged “electrons”.

  10. Thomson Model • He proposed a model of the atom called the “PlumPudding” model.* • Atoms were made from a positively chargedsubstance with negatively charged electrons scattered about, like raisins in a pudding. • Aka: “raisin pudding model”

  11. Millikan (1909) • OIL DROP experiment • Measures charge on electron + -

  12. Rutherford’sgold foilexperiment(1911) Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick) • Result: most of space occupied by atom is empty space! Except for small, dense, positively charged NUCLEUS. • Positive particles named PROTONS all protons are in the nucleus.

  13. Rutherford Nuclear Model • all of an atom’s positively charged particles are contained in the nucleus. The negatively charged particles were scattered outside the nucleus.

  14. Bohr Model • In 1913, the Danish scientist Niels Bohr proposed that each electron is in a specificenergy level.

  15. Bohr Model • According to Bohr’s atomic model, electrons move in definite orbitsaround the nucleus, much like planets circle the sun.

  16. Wave Model Louis DeBroglie (1924) e’s have wavelike properties

  17. Wave Model Erwin Schrodinger (1927) Quantum mechanics model e’s location probability

  18. Chadwick (1932) • Discovers particle with same mass as proton but no charge …the NEUTRON

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