1. Democritus/Leuccippos 400 BC
Greek philosophers
Matter is composed of atoms; between atoms lies empty space
“Atomos” – in Greek means uncuttable
Indivisible atoms
Atoms always will be, in motion

2. Aristotle Greek Philosopher All matter composed of the four elements:
Earth
Fire
Air
Water
He was wrong, but b/c he was well respected his idea was trusted over Democritus.

3. John Dalton Designed the ‘Billiard Ball Model’
Atoms are solid sphere shapes
Matter is made tiny indivisible particles
called atoms, which cannot be destroyed.
Each element is made of unique atoms and cannot transform to become other elements.
All atoms of an element are the same but different from atoms of other elements.
Elements combine in whole number ratios with other elements to form compounds

4. John Dalton Limitations: Did not know about subatomic particles
Did not know about nuclear reactions
Did not know about isotopes

5. J.J. Thomson Designed the Plum Pudding Model
Atom contains negative and positive charges suspended throughout.
Cathode Ray Tube experiment:
Discovered the electron
Theory that the electrons are suspended throughout the atom and help in place by an opposite charged area.
Predict the electrons contain the atoms mass
Agrees the atom is mostly empty space.

6. J.J. Thomson Limitations: Did not know placement of particles.
Believed the electrons contained the
mass of the atom

7. Rutherford Created the Nuclear Model Gold Foil Experiment Limitations:
Discovered the “nucleus” small, dense,
positively charged center of the atom
Proved the atom was mostly empty space.
Electrons were moving around the
nucleus of the atom
Limitations:
Could not explain the electron
paths

8. Neils Bohr Created the Planetary Model
Electrons orbit the nucleus in “fixed” orbits
They can jump up a level when energy is added
Light is emitted as the electron goes back down to the ground state

9. James Chadwick
Previously worked with Rutherford in looking at subatomic particles
Discovered the neutron
Neutral with a mass equal to a proton
neutron

10. Louis de Broglie Came up with the idea of wave-particle duality
Atoms/particles can act as a wave
λ = hv

11. Heisenberg
Uncertainty principle - Impossible to know the exact position and velocity of an object (electron) simultaneously
The more accurate you measure one property, the less accurate the other becomes

12. Schrodinger Quantum Model Electron cloud model
Orbital – where you will most likely find an electron
Electrons are in constant motion
Electron cloud model
Accepted today

13. Millikan
Discovered the charge of an electron and the mass of it as well using the Oil Drop experiment
balancing gravitational force with the upward buoyant and electric forces on tiny droplets of oil suspended between two metal electrodes
Also worked with the concept of Photoelectric effect