1. Chapter 4
Atoms

2. The Development of Atomic Theory

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

4. 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.

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

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

7. Cathode Ray Tube Experiment

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

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

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

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

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

13. Erwin Schrödinger (1926) Electron cloud Quantum mechanics
orbital: region around the nucleus where e- are likely to be found
Quantum mechanics
Mathematical equations

14. 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

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

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

17. Parts of positive and negative charges