1. Ch 3 Atomic Theory Models of the Atom

2. Pre-Dalton Atom Concepts
The Greek classical elements are Fire, Earth, Air, and Water.
Chinese Philosophy had 5 elements that composed nature.
Democritus was a Greek philosopher who lived between B.C. He developed the concept of the 'atom', Greek for 'indivisible'. Democritus believed that everything in the universe was made up of atoms, which were microscopic and indestructible.

3. Ch 3 Atomic Theory
John Dalton
( )
Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior
all atoms of an element were identical
compounds consisted of atoms of different elements combined together
Compounds have constant composition because they contain a fixed ratio of atoms
chemical reactions involved the rearrangement of combinations of those atoms

4. Dalton’s 3 Laws Law of Definite Proportions
John Dalton
( )
Law of Definite Proportions
Atoms combine in simple, whole number ratios to form compounds
Law of Multiple Proportions
The mass percentage of elements in a compound is fixed
Law of Conservation of Mass
The mass of the reactants is equal to the mass of the products in any chemical reaction.

5. J. J. Thomson (1897)
Thomson’s model had the atom as a sea of positive charge interspersed with negative “corpuscles” to neutralize the atom.
It was called the “Plum Pudding” model.
His proof was that atoms could be deflected in a magnetic field due to their charge.

6. Cathode Ray Tube
Thomson accurately measured the ratio of mass to charge, but not the actual charge on the electron.

7. Robert Millikan (1902)
By balancing the electrical force on charged oil droplets in an electric field with the force of gravity, Robert Millikan was able to independently determine the charge of the electron (-1.6x10-19 C) in his Oil Drop Experiment.

8. Millikan Oil Drop Experiment

9. Ernst Rutherford (1911)
Rutherford bombarded atoms (gold Foil) with Alpha particles (He2+).
If the atom was as Rutherford had predicted, they should pass through being slightly deflected.
However, some alpha particles were deflected back indicating a dense, positive center of the atom.

10. Gold Foil Experiment

11. Rutherford’s Model of the Atom

12. James Chadwick
In 1932, Chadwick made a fundamental discovery in the domain of nuclear science: he proved the existence of neutrons - elementary particles devoid of any electrical charge. In contrast with the helium nuclei (alpha rays) which are charged, and therefore repelled by the considerable electrical forces present in the nuclei of heavy atoms, this new particle need not overcome any electric barrier and is capable of penetrating and splitting the nuclei of even the heaviest elements. Chadwick in this way prepared the way towards the fission of uranium 235 and towards the creation of the atomic bomb.