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The History of Atomic Theory

The History of Atomic Theory. Mr Nelson. Democritus. 400 BC. The Greek philosopher Democritus began the search for a description of matter more than 2400 years ago.

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The History of Atomic Theory

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  1. The History of Atomic Theory Mr Nelson

  2. Democritus 400 BC • The Greek philosopher Democritus began the search for a description of matter more than 2400 years ago. • He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?

  3. Atomos • His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. • This piece would be indivisible. • He named the smallest piece of matter “atomos,” meaning “not to be cut.”

  4. Developing Modern Atomic Theory Democritius’ ideas were largely ignored and forgotten for more than 2000 years!

  5. Why? • The eminent philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory. Aristotle and Plato favored the earth,fire, air and waterapproach to the nature of matter. The atomos idea was buried for approximately 2000 years.

  6. Dalton’s Model • In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.

  7. Dalton’s Model • 1803 • Dalton’s Model was that atoms are indivisible particles.

  8. Dalton’s Theory • He deduced that all elements are composed of atoms. • Atoms of the same element are exactly alike. • Atoms of different elements are different. • Compounds are formed by the joining of atoms of two or more elements.

  9. Dalton’s theory became one of the foundations of modern chemistry.

  10. J. J. Thomson • In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles.

  11. Thomson Model • Thomson studied the passageof an electric current through a gas. Using a CRT. • As the current passed through the gas, it gave off rays of negatively charged particles.

  12. Thomson Model Where did they come from? • This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from?

  13. Thomson • Thomson concluded that the negative charges came from within the atom. A particle smaller than an atom had to exist. • Thomson called the negatively charged “corpuscles,” today known as electrons. • Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom. • But he could never find them. They were discovered by his student.

  14. 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.

  15. Rutherford’s Gold Foil Experiment • In 1908, the English physicist Ernest Rutherford, a student of J. J. Thomson, began work on his gold foil experiment.

  16. Rutherford’s Hypothesis • Rutherford was trying to verify Thomson’s model. • He expected positively charged alpha particles to go straight through a piece of very thin gold as the electrons were distributed throughout the atom.

  17. What Happened • Most alpha particles passed straight through the gold foil • A small percentage (1/8000) were deflected at large angles or returned to the source • http://micro.magnet.fsu.edu/electromag/java/rutherford/

  18. Rutherford’s Experiment • There are 2 reasons alpha particles deflected • Density of the nucleus • Repelling charges

  19. Rutherford’s Conclusion • Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged alpha particles. • He called the center of the atom the “nucleus” • The nucleus is tiny compared to the atom as a whole. • This could only mean that the gold atoms in the sheet were mostly open space. Verified by the high percentage of particles that went straight through.

  20. Rutherford’s Nuclear Model • Rutherford reasoned that all of an atom’s positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge.

  21. Neils Bohr • Was a Jewish Scientist in Copenhagen during the onset of WWII • Hitler was interested in his research of the atom. • He was moved to the US to protect his knowledge.

  22. Bohr’s Explanation • Bohr thought that an electron travelled in a specific orbit at a certain distance from the nucleus called an energy level and had specific amounts of energy. • Worked well for Hydrogen and Helium

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