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The History of Atomic Theory

ACTIVITY: What do you remember about Atoms?

  • Atoms are made out of protons, electrons and neutrons

  • In an atom, the amount of protons and electrons are always equal

  • Atoms are a pure/type of substance

  • Atomic mass, the weight of the atom

  • Atomic number, the number on the periodic table

What is a Model

  • Uses familiar ideas to explain unfamiliar facts observed in nature

  • Can be changed as new information is collected

Democritus

Greek philosopher who began the search for a description of matter.

His question:

<aside> Could Matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided.

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His Theory

Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes.

“Atomos” = not to be cut

Why was his Theory so Important

The eminent philosophers Aristotle and Plato favoured the 4 element approach to the nature of matter

Dalton’s Model (early 1800s)

English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms. He was the one who revived the idea of atoms.

The 5 Postulates Dalton’s Atomic Theory

Postulate: Point of a theory, idea taken to be true.

  1. All matter is made of indivisible particles called atoms

  2. Atoms of the same element are similar to each other (in terms of shape and mass)

  3. Atoms of different elements differ from each other in shape and mass

  4. Atoms can neither be created nor destroyed

  5. Atoms combine in whole number rations to form compounds

Thomson’s Plum Pudding Model

Atoms are made from a positively charged substance with negatively charged electrons scattered about.

First to identify the negative charges within the atom

Experiments leading to the Thomas Model

  • He passed a current through gas

  • Demonstrated that the “cathode rays” coming from the gas were negatively charged particles

  • The negatively charged particles must be apart of atoms