Chapter 4.1 – The Development of Atomic
Theory
1. Democritus –
- Greek philosopher, 4th century BCE
- suggested universe was made of indivisible units called
atomos – means unable to be cut or divided
- did not have any evidence
- forgotten until 1700’s
Chapter 4.1 – The Development of Atomic
Theory
2. John Dalton –
- English school teacher, 1808
- atoms could not be divided
- all atoms of a given element were exactly alike
- atoms of different elements could join to form compounds
- developed the law of definite proportions – a chemical
compound always contains the same elements in exactly the
same proportions by mass
Chapter 4.1 – The Development of Atomic
Theory
3. J.J. Thomson –
- British scientist, 1897
- studied cathode rays
- experiments suggested cathode rays were negatively charged
particles from inside an atom
- discovered electrons – subatomic particles with a negative
charge
- proposed the plum-pudding model – electrons are spread
throughout the atom
Chapter 4.1 – The Development of Atomic
Theory
4. Ernest Rutherford –
- British scientist, 1911
- proposed most of the mass of the atom was concentrated at the
atoms center
- conducted gold-foil experiment
- discovered the nucleus – the atom’s central region made up of
protons and neutrons
- Rutherford’s model – negative electrons orbit the positive
nucleus