A Historical Perspective Atomic Theory A Historical Perspective

The Old Classification of Matter earth warm wet fire water cold dry wind

The first “atomic theory” Democritus (c. 400 B.C.) “All matter exists as tiny, indivisible particles called ‘atmos’.” atmos is Greek for “indivisible”

The Phlogiston Theory All substances contain some quantity of an invisible, fluid-like substance called “phlogistos”. This phlogistos enables a substance to burn. When the phlogistos was completely consumed, the burning would stop …

enter Antoine Lavosier … Often referred to as the “father of modern chemistry” developed the metric system disproved the phlogiston theory Wrote what is now called the Law of Conservation of Mass c. 1772

some important “discoveries” The Law of Definite Proportions: No matter how much of a compound you have, the component elements account for certain exact percentages by mass. The Law of Multiple Proportions: If two or more compounds are composed of the same elements, the ratios of their respective masses is always a ratio of small whole numbers.

John Dalton the Atomic Theorie All matter is composed of tiny, indivisible particles called atoms. The atoms of a single element are all identical. Atoms of different elements are fundamentally different. Atoms of different elements combine in whole-number ratios to form compounds. During chemical reactions, atoms are combined, separated, or rearranged. However, the identity of the individual atoms is maintained.

still more to note … Investigations are conducted to explore new phenomena, to check on previous results, to test how well a theory predicts, and to compare different theories.

a very significant discovery … the cathode ray tube

The work of J.J. Thomson Demonstrated that the cathode ray was actually a stream of particles that had a negative charge. He called these particles “electrons” for electric particles. This is 1897. Before the first car. Before the Wright brothers first flight.

Under the heading of “Chance Favors the Prepared Mind” … Thirty years earlier (~1865) Goldstein has assembled a cathode ray tube with holes (he called them canals) in the cathode. He observed and reported rays coming through the canals and he called them “canal rays”. Unfortunately, he has no idea of the significance of this discovery and draws no conclusions…

doing the right thing … Thomson and his research team re-built Goldstein’s apparatus and discovered that the canal rays were actually streams of positively charged particles. They named these particles “protons” from the Greek word protos which means first. This was to honor Goldstein for having been there first. (even though he did not know it!!)

meanwhile in Germany … Wilhelm Roentgen discovered that the cathode rays caused both glass and metals to emit their own rays that seemed to ruin photographic film. Having no earthly idea where these rays were coming from, he simply called them “unknown rays” or X-Rays

several years later in France… Henri Becquerel accidentally discovered that certain metal ores would expose (and ruin) film just like x-rays would. However, these ores did not need any additional energy source to do this. One of Becquerel’s students used the term radioactivity to describe this spontaneous emission of particles and/or radiation. Since then, any element that spontaneously emits radiation is said to be radioactive

So where are we at this point??? It’s the early 1900’s and we have the ability to shoot “x-rays” (or other forms of radiation) at things.

and then there was Ernie … Ernest Rutherford conducted what is now called the “gold-foil” experiment.

Analysis of the Gold-Foil experiment:

Conclusions of the Gold-Foil experiment: Every atom has a very small but very dense center structure that was referred to as the nucleus. This nucleus has a positive charge which means that all of the protons are gathered in this structure. The nucleus is surrounded by a region called the electron cloud. This electron cloud is almost completely empty space.