History of the Atomic Model

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)

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

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

Dalton’s Shortfall

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

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

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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

Gold Foil Experiment

• http://www.mhhe.com/physsci/chemistry/animations/chang_2e/rutherfords_experiment.swf

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

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

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

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Summary of Atomic Models1) Dalton’s “Billiard ball” model (1800-1900)

Atoms are solid and indivisible.

2) Thomson’s “Raisin bun” model (1900)Negative electrons in a positive framework.

3) 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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