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Section 2: Atomic Discoveries

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1 Section 2: Atomic Discoveries
Chapter 4: Atoms and Elements Section 2: Atomic Discoveries

2 Learning Goals Explain how the experiments of Thomson, Rutherford, and Chadwick led to the development of the nuclear theory of the atom. Describe early atomic models.

3 Solid Sphere Solid Sphere Model: Democritus Dalton

4 J. J. Thomson J. J. Thomson (1856–1940)
In 1897, Thomson performed his famous cathode-ray tube experiments.



7 J. J. Thomson Electricity was passed through a vacuum sealed tube.
When charged plates were brought close to the tube, the beam of electricity reacted. Moved towards the positively charged plate Moved away from the negatively charged plate

8 J. J. Thomson Based on his experiments, Thomson discovered the following: Electrons are negatively charged. Electrons are much smaller and lighter than atoms.

9 J. J. Thomson Electrons are uniformly present in many different kinds of substances. He proposed that atoms must contain positive charge that balances the negative charge of electrons.

10 J. J. Thomson Plum Pudding Model J.J. Thomson
Negatively charged electrons were held in a sphere of positive charge

11 Ernest Rutherford Ernest Rutherford (1871-1937)
In 1909, Rutherford performed his famous gold foil experiment.




15 Ernest Rutherford Tiny particles called alpha-particles were directed at a thin sheet of gold foil. Most particles passed straight through the foil, but some were deflected at large angles.

16 Ernest Rutherford Expected result of Rutherford’s gold foil experiment: If the plum pudding model were correct, the alpha- particles would pass right through the gold foil with minimal deflection.

17 Ernest Rutherford Actual result of Rutherford’s gold foil experiment:
A small number of alpha- particles were deflected or bounced back.

18 Ernest Rutherford Based on his experiments, Rutherford discovered the following: Most of the atom’s mass and all of its positive charge are contained in a small core called the nucleus.

19 Ernest Rutherford Most of the volume of the atom is empty space through which the tiny, negatively charged electrons are dispersed. The number of negatively charged electrons outside the nucleus is equal to the number of positively charged particles (protons) inside the nucleus, so that the atom is electrically neutral.

20 Ernest Rutherford Planetary Model Ernest Rutherford
A small, dense positively charged nucleus is orbited by electrons.

21 James Chadwick James Chadwick (1891-1974)
In 1919, Chadwick and his mentor, Rutherford, kept finding that the atomic mass of an atom was larger than the atomic number of that atom.

22 James Chadwick Chadwick suggested that there could be an additional particle in the nucleus that had mass, but no charge. These particles were called neutrons

23 James Chadwick Nuclear Model Ernest Rutherford James Chadwick
99.9% of the atoms mass is concentrated in a small dense nucleus that contains protons and neutrons.

24 James Chadwick The dense nucleus makes up more than 99.9% of the mass of the atom but the nucleus occupies only a small fraction of an atoms volume The electrons are distributed through a much larger region but don’t have much mass.

25 James Chadwick A single grain of sand composed of solid atomic nuclei would have a mass of 5 million kg. Astronomers believe that black holes and neutron stars are composed of this kind of incredibly dense matter.

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