1. Chapter 2 Section 1
8th Grade

2. Section 1: Models/History of the Atom
Greek philosophers devised a theory of atoms, or tiny particles. (fourth century BC)
John Dalton( ) – Combined the idea of elements with the Greek theory of the atom. Thought of the atom as a hard sphere that was the same throughout.
1. Matter is made up of atoms.
2. Atoms cannot be divided into smaller pieces.
3. All atoms of an element are exactly alike.
4. Different elements are made up of different atoms.

3. Dalton’s Model
Hard Sphere that was the same throughout.

4. C. J.J. Thomson – (1897)discovered negatively charged particles (electrons) which are a part of every atom.
Thomson revised Dalton’s model to include a sphere with a positive charge and negatively charged electrons spread evenly within the positive charge.
The negatively charged electrons and the positive charge in the sphere neutralized each other.
Earnest Rutherford – (1911)tested Thomson’s model, which was found to be an inaccurate model of the atom.
1. Shot alpha particles at gold foil. If Thomson was right, the alpha particles should have gone through. Some bounced back from the large positive nucleus

5. Thomson’s Model
Positive sphere with negative charges throughout.

6. Rutherford’s Model
Large positive nucleus

7. F. Chadwick – (1932) confirmed existence of neutron.
E. Neils Bohr (1913)
1. Electrons orbit nucleus in energy levels.
2. Electrons can only be in energy levels
F. Chadwick – (1932) confirmed existence of neutron.
G. The Electron Cloud Model - (modern) explains the unpredictable wave behavior of electrons, which could be anywhere in the area surrounding the nucleus.

8. Bohr Model of the Atom
Electrons orbit the positive nucleus in energy levels.

9. Electron Cloud Model