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Historical Development of an Atom

Historical Development of an Atom. Democritus 470-380 B.C. Democritus was a Greek philosopher (470-380 B.C.) who is the father of modern atomic thought. He proposed that matter could NOT be divided into smaller pieces forever.

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Historical Development of an Atom

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  1. Historical Development of an Atom

  2. Democritus 470-380 B.C • Democritus was a Greek philosopher (470-380 B.C.) who is the father of modern atomic thought. • He proposed that matter could NOT be divided into smaller pieces forever. • He believed that matter was made of small, hard, particles, that he called “atomos”

  3. John Dalton 1807 • He built upon Democritus’s atomic theory. • Matter is made up of atoms • Atoms cannot be divided into smaller pieces. • All the atoms of an element are exactly the same • Different elements have different kinds of atoms • Compounds form by combining atoms

  4. John Dalton 1807 • Model Drawing • Solid throughout • Same throughout

  5. J.J. Thompson 1898 • Particles smaller than the atom exists • Discovered the electron, which is negatively charged • If atoms have negative particles, then atoms must also have positive particles.

  6. J.J. Thompson 1898 • Model Drawing • Positive and negative particles

  7. Ernest Rutherford 1911 • Rutherford performed the gold foil experiment. • In his experiment a radio active beam emitted (+) alpha particles toward gold foil. • Most particles went through, some were deflected, and others bounced back! • This showed that atoms have a very dense (+) center.

  8. Ernest Rutherford 1911 • Rutherford showed that atoms have (+) particles in the center and are mostly empty space. • He called the center of the atoms the nucleus. • All the mass of an atom and its positive charge are located in the nucleus.

  9. Ernest Rutherford 1911 • Model Drawing • Positive nucleus • Negative particles surround nucleus

  10. Niels Bohr 1913 • Niels Bohr improved on Rutherford’s model. • He proposed that electrons move around the nucleus in specific layers or shells. • Every atoms has a specific number of electron shells.

  11. Niels Bohr 1913 • Electrons move in orbits around the nucleus • 1st energy level = 2 electrons • 2nd energy level = 8 electrons • 3rd energy level = 18 electrons • 4thenergy level = 32 electrons

  12. Niels Bohr 1913 • Model Drawing • Electrons orbit the positive nucleus on energy levels

  13. James Chadwick 1932 • Chadwick discovered neutrons • He worked with Rutherford and discovered particles with no charge • He called those particles neutrons • Neutrons are found in the nucleus of an atom.

  14. Chadwick 1932 P n n n Pnnnnppp

  15. Electron Cloud ModelCurrent • Electrons do not follow fixed orbits, but tend to occur more frequently in certain areas around the nucleus • it is impossible to know where an electron is at any given time

  16. Electron Cloud ModelCurrent • Model Drawing • Electrons are located in clouds

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