1. History of the Atomic Model

2. Early Greek Theories Aristotle (350 B.C.) 4 Elements
Democritus (400 B.C)
Atoms and a void (empty space)
Atoms are indivisible

3. Dalton’s Billiard Ball Model (1805)
All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine in constant ratios to form compounds.
Atoms are rearranged in reactions, but are not created nor destroyed.
Matter is composed of indestructible, indivisible atoms

4. Thomson’s Raisin Bun Model (1897)
Materials, when rubbed, can develop a charge difference.
This electricity was called “cathode rays”
These rays have a small mass and are negatively charged.
Thomson noted that these negative subatomic particles (electrons) were a fundamental part of all atoms.

5. Rutherford’s Nuclear Model
Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive substance
path of invisible -particles
Most particles passed through. So, atoms are mostly empty.
Some positive -particles deflected or bounced back!
Thus, a “nucleus” is positive (protons) & holds most of an atom’s mass.

6. Limitations to Rutherford’s Model
Orbiting electrons should emit light, losing energy in the process
This energy loss should cause the electrons to collapse into the nucleus
However, matter is very stable, this does not happen
an electron in motion around a central body MUST continuously emit radiation (rainbow of light_, should also then lose energy (shrink). Either our understanding of laws of physics is flawed or rutherford or both

7. Bohr’s Planetary Model
Electrons orbit the nucleus in energy “shells”
An electron can travel indefinitely within an energy level without losing energy
The greater the distance between the nucleus and the energy level, the greater the energy level
An electron cannot exist between energy levels, but can move to a higher, unfilled shell if it absorbs a specific quantity of energy, or to a lower, unfilled shell if it loses energy (quantized)
When all the electrons in an atom are in the lowest possible energy levels, it is in its ground state.

8. Bohr’s Planetary Model
An atom becomes excited when one of its electrons absorb energy in the form of photons
If enough energy is absorbed then the electron can make a quantum leap to the next energy level, if there is room
When the electron returns to a lower energy state the energy is released in the form of a photon, which we see as visible light
The energy of the photon determines its wavelength or color
Each element has its own frequencies of color, so it emits its own distinctive glow

10. 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)