1. Atomic Theory

2. John Dalton First to develop a theory on the structure of an atom
Based on 5 major principles

3. Dalton’s 5 Principles 1) All matter is made of indivisible atoms
2) All atoms of a given element
are identical in their
chemical and physical
properties
3) Atoms of different elements
have different chemical and
physical properties
Dalton’s notation of elements:
21 = water, 22 = ammonia,

4. Dalton’s 5 Principles
4) Atoms of different elements combine in simple, whole number ratios to form compounds
Ex: 2 H + 1 O  H2O
5) Atoms cannot be subdivided, created, or destroyed when they combine, separate or rearrange in reactions
*Law of Conservation of Mass

5. Problems with Dalton’s ideas
JJ Thomson was able to prove that atoms were made of smaller particles that contained charge
Sent electricity through a cathode ray tube – gas inside glowed

6. J.J. Thomson
Used a cathode ray tube to prove the small particles must have a NEGATIVE charge = Electron
Electric beam bent with a magnet
Cathode Ray Tube

7. J.J. Thomson
Atoms were electrically neutral, so there must be a + particle to cancel out the – charge from the electron
1897 – developed the “Plum Pudding” model of an atom
Positive “ball” containing electrons

8. Robert Millikan - 1909 Oil-drop Experiment
Determined the charge on an electron to be 1.602 × 10−19 coulomb
Determined mass of electron to be 1/2000 of a proton

9. Ernest Rutherford Student of Thomson
Conducted the Gold Foil Experiment in 1909 to disprove the Plum Pudding model

10. The Gold Foil Experiment
Sent a beam of + charges (alpha particles) through a very thin piece of gold foil
Angles of deflection were measured

11. Gold Foil - Conclusion
Since most alpha (+) particles passed straight through, most of the foil must actually be “empty regions” – not a solid + sphere like Thomson believed
Positive charge and most of the mass of an atom must be found in the core/center = NUCLEUS

12. Rutherford’s Model of an Atom

13. Niels Bohr
There are only certain discrete orbits allowed in an atom and no others
Electrons must be in one of these orbits

14. Erwin Schroedinger
1926
Modified atomic structure to include electron’s location in a “cloud” or “shell” instead of a predictable orbit

15. James Chadwick 1932 – discovered the neutron
A neutron contained no charge and a mass equivalent to that of a proton