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Development of the Atomic Theory

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Presentation on theme: "Development of the Atomic Theory"— Presentation transcript:

1 Development of the Atomic Theory

2 Definition of an Atom An atom is the smallest particle into which an element can be divided and still be the same substance.

3 Democritus Proposes the Atom
Can you keep cutting something in half forever?

4 Aristotle disagrees Democritus’s ideas were largely ignored for centuries

5 John Dalton Created an Atomic theory based on experiments.

6 Dalton’s Atomic Theory
Atoms are small particles that cannot be created, divided, or destroyed Atoms of the same element are exactly alike. Atoms join with other atoms to make new substances.

7 J. J. Thomson Discovered that atoms can be divided into smaller particles Discovered one such particles called the electron.

8 Cathode-Ray tube Experiment
When particles were passed through a magnet they were attracted to the positive end, causing the beam of particles to bend downward. Thompson inferred that there must two different sections of the atom, one positively charged and the other negative. He called the negative portion of the atom the electron.

9 The Thompson Model 1. Electrons are embedded in a positively charged sphere, much like chocolate chips in cookie dough.

10 Ernest Rutherford Discovered the Nucleus of the Atom
At the center of an atom is a small dense positively charged nucleus. The atom is mostly empty space through which electrons travel

11 Gold Foil Experiment

12 The Gold Foil Experiment
Shot a beam of particles at a thin piece of gold foil (so thin it was only 1 atom thick). Behind the foil was a piece of photographic film Most of the particles passed right through the foil. But, occasionally one would be reflected right back at him, because it hit the hard dense nucleus.

13 Neils Bohr Suggested that electrons travel around the nucleus in definite paths States that electrons can jump between energy levels

14 Bohr Model of the Atom

15 Modern Theory Electrons do not travel in definite paths
Instead they reside in electron clouds Electron clouds are regions around the atom in which the electrons likely to be found

16 Modern Theory


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