1. The Idea of the Atom

2. Democritus (460-370 B.C.) Greek philosopher
Believed the world was made of two things – empty space and tiny particles
Called the tiny particles “atomos”- meaning indivisible

3. John Dalton (1802) 4 Postulates:
1. Each element is composed of extremely small indivisible particles (same model as Democritus)
2. Determined that all atoms of a given element are identical to each other, but they differ from those of any other element.
3. Law of Conservation of Matter= matter is not created nor destroyed in a chemical reaction
4. Law of Definite Proportions= A given compound always has the same ratio and type of atoms. Ex: water H2O is always 2Hydrogens &1 Oxygen

4. JJ Thomson (1904)
Plum Pudding Model (looks more like a chocolate chip cookie, with negative “chips” and positive dough)
Discovered electrons
Experiment with cathode ray tube
Particles move toward positive end, therefore it must contain negatively charged particles
Disproved that the atom was indivisible (it could be broken into parts)

5. Cathode Ray Tube

6. Robert Millikan (1911) Oil drop Experiment
Able to stop a drop of oil in mid air using magnetic field
Could then measure the charge on an electron & calculated the mass of an electron!!
Won the Nobel Prize in Physics (1923)

7. Oil drop Experiment
What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must have. By varying the charge on different drops, he noticed that the charge was always a multiple of -1.6 x C, the charge on a single electron. This meant that it was electrons carrying this unit charge.

8. 1911-Rutherford New Atomic Theory
Discovered that the atom was mostly empty space with a dense positively charged nucleus
Gold Foil Experiment
Disproved the Plum Pudding Model
(btw he was a grad student of Thomson)

9. Rutherford’s Experiment

10. Thomson versus Rutherford

11. Niels Bohr (1913) Planetary Atomic Model Electrons moved in an orbit
Each orbit had a definite energy
Neils Bohr chillin with his buddy Albert

12. Electron Cloud Model The model now is adapted from Bohr’s model
Electrons do not follow paths
The electron “cloud” shows where the electron is most likely to be
Where you have a 90%
chance of finding
the electron

13. Heisenberg (1927)
Concluded that the only way to know where an electron is, is to hit it with a photon and by doing so you would change it’s location.
Known as “Heisenberg’s Uncertainty Principle”

14. Louise deBroglie- 1924 Dual Nature of Matter
Determined that electrons travel as waves. Known as “Matter Waves”
Allowed us to build electron microscopes which can see large atoms

15. 1926- Schrodinger Developed a mathematical equation to explain the
wave-particle duality
Began the field of Quantum Mechanics (aka Quantum Physics)

16. Today: Quantum Mechanical Model
Developed from Heisenberg, de Broglie & Schrodinger

17. James Chadwick Discovered the 3rd and final subatomic particle-neutron
Has no charge but does have the same mass as a proton
BTW- He was a POW in WWI,
when released he returned
to study under Rutherford

18. History of Atomic Models
Dalton 1803
Thompson 1897 “plum pudding”
Rutherford 1909
Bohr 1913
“planetary model”
Quantum Mechanical Model
Electron-cloud model

19. Atomic Theory Timeline ’s
James Chadwick
John Dalton
J.J. Thompson
Antoine LaVoisier
Ernest Rutherford
Niels Bohr
Joseph Proust
Robert Milliken
Law of
conservation
of mass
1897 – Cathode
Ray Experiment
Neutron discovered
1803 – Law of
Multiple proportions
1912 – Gold Foil
experiment
Oil drop
experiment
1799 – Law
Of Definite
Proportions