1. History of the Atomic Model

2. Greeks (300 B.C)
Hypothesized that matter cut into smaller and smaller pieces would eventually reach the atom (literally meaning indivisible)
Thought there were 4 elements: Air, Fire, Earth, Water
Findings based on reason, not
empirical evidence
(experimentation)

3. Dalton’s “Billiard Ball” Model (1808)
Atoms are small indestructible,
indivisible spheres
All matter is made of atoms or combinations of atoms (in specific proportions/ratios)
All atoms of one element are identical
Matter cannot be created or destroyed, only changed in form

4. Dalton’s Laws
Law of definite composition: elements combine in a characteristic mass ratio
Law of multiple proportions: there may be more than one mass ratio
Law of conservation of mass: total mass remains constant

5. Dalton’s Shortfall Dalton’s model did not explain the behaviour of
substances (why atoms
combined in ratios)

6. Thomson’s “Raisin Bun” or “Plum Pudding” Model (1897)
Discovered a negatively charged particle, the electron using cathode ray experiments
Atoms consisted of a positively charged sphere embedded with negatively
charged electrons
Overall charge of the atom
is neutral
Charged particles could
exist if electrons were
added or removed

7. Rutherford’s “Electron Cloud” Model (1911)
Discovered the proton by using cathode
ray tubes and found the smallest possible
positive charge from ionized hydrogen
Gold Foil Experiment:
Aimed alpha particles at thin metal foil
1/8000 alpha particles were deflected significantly
Inconsistent with Thomson’s Model
Conclusion:
Atoms are made up mainly of empty space, with small concentrated charge around the center

8. Gold Foil Experiment

9. Rutherford’s Limitations
A nucleus made of entirely positive charges should fly apart
Did not explain the total mass of the atom
Solution: In 1932 the neutron was discovered by Chadwick by bombarding nuclei with alpha particles and noticing a difference in their mass and that of the number of protons they contained
Orbiting electrons should emit light and constantly lose energy and then collapse
However, matter is very stable and this does not happen

10. Bohr’s “Planetary” Model
Electrons have specific, allowable energy levels
Electrons moving within an energy level (shell) do not lose energy
An electron changes energy levels by emitting or absorbing a specific quantity (quantum) of energy in the form of light (photons)
When electrons in an atom are in the lowest possible energy levels, they are in its ground state

11. Bohr’s “Planetary” Model (cont.)
When an electron absorbs energy in the form of photons, the atom becomes excited
If enough energy is absorbed, the electron can leap to the next energy level
When the electron returns back to it’s original state, it releases energy in the form of a photon, which we see as visible light
The energy of the photon determines its wavelength or colour
Each element has its own frequencies of color

12. http://www. visionlearning. com/library/flash_viewer. php

13. Summary of Atomic Models
Dalton’s “Billiard ball” model ( )
Atoms are solid and indivisible.
Thomson’s “Raisin bun” model (1900)
Negative electrons in a positive framework.
Rutherford’s “Nuclear” model (~1910)
Atoms are mostly empty space.
Negative electrons orbit a positive nucleus.
4) Bohr’s “Planetary” model (~1920)
Negative electrons orbit a positive nucleus.
Quantized energy shells
5) Quantum Mechanical model (~1930)
Electron probabilities (orbitals)
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