1. the earliest atomic theories
Democritus to Dalton
the earliest atomic theories

2. Democritus’ Theory
Every substance is made up of tiny, indivisible (unbreakable) particles called atoms.
A piece of paper is made of paper atoms.
A leaf is made of leaf atoms.
Skin is made of skin atoms.
These atoms cannot be broken down any further.
Was there any science involved in this model?
NO! Democritus was a philosopher, not a scientist.

3. Before Dalton Antoine Lavoisier – Law of Conservation of Mass
Mass is never gained or lost in a chemical reaction.
Joseph Proust – Law of Constant Composition
A compound always contains the same elements in the same proportions, by mass.
For example:
Water is always H2O – 88.9% oxygen, 11.1% hydrogen
Hydrogen peroxide is H2O2 – 94.1% oxygen, 5.9% hydrogen

4. Dalton’s Atomic Model
Atoms are indivisible units of matter that are unique for each element, and combine to make various compounds.
A lump of gold is made of tiny gold pieces (atoms).
Carbon dioxide is made of tiny carbon and oxygen pieces (atoms) combined together.
These atoms cannot be broken down further.
Was there any science involved in this model?
YES! John Dalton did many scientific experiments, especially with gases, to formulate his ideas.

5. Dalton’s Atomic Theory of Matter
Four Postulates:
All matter is composed of extremely small particles called atoms.
Atoms of the same element are identical, and atoms of different elements are different from those of any other element.
Atoms are neither created nor destroyed in any chemical reaction.
Atoms combine in definite proportions (the same proportion) to form compounds. Ex. CO vs. CO2

6. J. J. Thompson’s
Plum Pudding Model

7. J. J. Thompson – 1898
Thompson took advantage of new technology – the cathode ray tube. Sketch the diagram on the next page.
Cathode “rays”/particles – negatively charged, very light (but they do have mass—Milliken’s Oil Drop Experiment), came from atoms.

9. Thompson’s Discovery
Thompson knew atoms were usually neutrally charged. So, he realized that if he could remove a negative particle, there must be a positively charged part also.

10. Thompson’s Conclusions
Thompson’s model of the atom is heterogeneous and solid, but NOT indivisible.
Electrons can be removed, so it can be divided.
The Plum Pudding Model – Solid sphere of positive charge with electrons spread through it like raisins or plums in a plum pudding.