Early Atomic Theories


 
 Previously Covered / Dalton / Thomson / Rutherford / Bohr
 

 

Previously Covered Concepts

It is recommended that the following concepts are introduced before talking about atomic theory:

Before talking about the different theories of the atom, you may want to have students come up with their own models of the atom.  As we talk about each theory of the atom and introduce new evidence, students can update their own models and see how it compares to that of the historical evolution. Questions to ask may include: Make sure they give their reasoning for each characteristic.

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John Dalton

History: 

John Dalton was an English school teacher that lived in the early 1800's.  At the age of 12, he began teaching.  He came up with his theory of the atom based on experimental data of the time.
 
Evidence for his model:
 
Experimental data shows:

Theory:  
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 J.J. Thomson

History:
 John Joseph Thomson was a physics professor at Cambridge University, in England.  He created his "plum pudding" theory based on cathode-ray tube experiments that he conducted.  A cathode-ray tube is a sealed tube that contains a low pressure gas.  At either end of the tube is an electrode.  When powered, a beam of charged particles originates from the negative electrode, or the cathode as it is called.

Evidence for his model:

Theory:
Based on the evidence, Thomson came up with his "Plum Pudding" model of the atom: To help students understand the nature of these "moving charged particles", or electrons, have them try The Pith Ball Experiment
 
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 Ernest Rutherford

History:
Rutherford was born and educated in New Zealand.  From 1898 to 1907, he was a physics professor at McGill University in Montreal.  From 1907 to 1919, he worked at the University of Manchester, and afterwards, finished hiss brilliant career at Cambridge University.  He was awarded the Nobel Prize in Chemistry in 1908, and knighted in 1914.

His greatest contribution to science was most likely his model of the atom which he devised after performing his famous gold foil experiment.  Shooting alpha particles, small positively charged particles, at a thin sheet of gold foil, it was expected that the particles would pass through the sheet relatively unaffected.  This would have been consistent with the J.J. Thomson's widely accepted atomic model.  It was a surprise when some alpha particles actually bounced back from the gold sheet.  This was as unexpected ass if they had shot a bullet at a piece of tissue paper, and the bullet had bounced back.

Interesting Fact:  Rutherford was a modern Alchemist.  He was the first person to transform one element into another.  By bombarding nitrogen atoms with alpha particles, he produced oxygen atoms.
 
Evidence for his model:

The Gold Foil Experiment:

Theory:
Based on the gold foil experiment, Rutherford made the following statements about the atom: James Chadwick, another scientist,   discovered a second nuclear particle while working with Rutherford.  This was a particle that had no charge and so was neutral.  It was called the Neutron.

 To re-enact the "Gold Foil Experiment", try this demo.

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Niels Bohr

History:
Bohr was born in Denmark in 1885.  He finished his doctorate at the age of 26, and later worked with Ernest Rutherford at the University of Manchester.  During the Second World War, Bohr acted as an advisor to the physicists working on the atomic bomb.  He did not, however, work on the project himself, and was opposed to the development of such a destructive force.

Rutherford and his colleagues had proposed that electrons orbit the nucleus in orbits, much like the planets orbit the sun.  This however raised a problem of energy loss.  If orbiting electrons eventually lost energy, they would spiral towards the nucleus and the atom collapsed.

At this point it was known that light was made up of discrete packages of energy, or quanta.  In the spectrum of light, each component color has a different frequency and so also has a different energy.  When put together, the energies form a spectrum of light that we see as white light.  When different elements are excited by an electric current, different line spectrum, or lines of colour from the visible spectrum, are given off.  Bohr used this idea to come up with his modification to the Rutherford theory of the atom.  Bohr won the Nobel Prize in 1922 for this idea, and is thought to have laid the groundwork for the modern quantum model of the atom.

Evidence for his model:

Theory:  ^Top