1. Chapter 4
Atoms

2. The Development of Atomic Theory Sections 1 and 3

3. Democritus (400 B.C.)
Proposed that matter was composed of tiny indivisible particles
Not based on experimental data
Greek: atomos

4. John Dalton (1808) British Schoolteacher Billiard Ball Model
based his theory on others’ experimental data
Billiard Ball Model
atom is a uniform, solid sphere

5. John Dalton Dalton’s Four Postulates
1. Elements are composed of small indivisible particles called atoms.
2. Atoms of the same element are identical. Atoms of different elements are different.
3. Atoms of different elements combine together in simple proportions to create a compound.
4. In a chemical reaction, atoms are rearranged, but not changed.

6. J. J. Thomson (1897) Cathode Ray Tube Experiments Discovered Electrons
beam of negative particles
Discovered Electrons
negative particles within the atom
Plum-pudding Model

7. J. J. Thomson (1897) Plum-pudding Model
positive sphere (pudding) with negative electrons (plums) dispersed throughout

8. Ernest Rutherford (1911) Gold Foil Experiment Discovered the nucleus
dense, positive charge in the center of the atom

9. Ernest Rutherford (1911) Planetary Model
dense, positive nucleus surrounded by negative electrons
daviddarling.info

10. Niels Bohr (1913) Energy Levels
electrons can only exist in specific energy states

11. Niels Bohr (1913) Bohr Model
electrons move in circular orbits within specific energy levels

12. Erwin Schrödinger (1926) Quantum mechanics Electron cloud model
electrons can only exist in specified energy states
Electron cloud model
orbital: region around the nucleus where e- are likely to be found

13. dots represent probability of finding an e- not actual electrons
Erwin Schrödinger (1926)
Electron Cloud Model
dots represent probability of finding an e- not actual electrons

14. James Chadwick (1932) Discovered neutrons
neutral particles in the nucleus of an atom

15. revision of Rutherford’s Nuclear Model
James Chadwick (1932)
Neutron Model
revision of Rutherford’s Nuclear Model

16. Parts of positive and negative charges