The History and Technical Facts of
THE ATOM
 



           This is a webpage designed to inform you of the history and technical facts of the atom.  The information that follows is taken from various sources.

      First of all, lets go back to the 1800's, where amongst the various experiments of the British physisist J.J Thompson, he was investigating the interior of the atom.  At the Cavendish Laboratory at Cambridge University, Thomson was experimenting with currents of electricity inside empty glass tubes. He was investigating a long-standing puzzle known as "cathode rays."
Shown below is a picture of a simple cathode ray tube.  His experiments prompted him to make a bold proposal: these mysterious rays are  streams of particles much smaller than atoms, they are in fact minuscule pieces of atoms. He called these particles "corpuscles," and suggested that they might make up all of the matter in atoms.  No one had thought that there might actually be something inside the atom. 


        THE ELELCTRON

    In 1897, Thompson discovered the first component part of the atom: the electron, a particle with a negative electric charge.  In 1904, he proposed an initial model of an atom, since nicknamed "Thompsons pudding".  He imagined the atom as a sphere full of an electrically positive substance mixed with negative electrons "like the raisins in a cake".
This is a picture of "Thompson's pudding" idea.


        THE DISCOVERY OF THE NUCLEUS

    In 1912, Rutherford (New Zealand physicist) discovered the atomic nucleus.  His new model of the atom showed that its positive electric charge and the majority of its mass were concentrated in an almost point sized nucleus.  The electron in an atom moves around this nucleus like planets around the Sun, and the attractive electric force (the - charge of the electron attracting the + charge of the nucleus) plays the role which gravity plays for the planets; which is where we get the name "atomic planetary model".  It is worth noting that in contrast to the atom of the Greeks, Rutherford's is neither indivisible (because it's a composite structure), nor is it solid as it contains mostly empty space: The distance nucleus-electrons is 100,000 times greater than the diameter of the nucleus itself (diameter of the nucleus = 10-15 units = 1 Fermi). 


        THE DISCOVERY OF THE NUCLEONS

    Rutherford understood that the nucleus is itself composed of nucleons. These nucleons are of two types:

     positively charged, it's a proton.
     neutrally charged, it's a neutron
The neutron was effectively discovered in 1932 by James Chadwick.



The planetary model of the atom has a serious deficiency. The electrons can emit light under certain conditions (in an electric light bulb for example); in doing so, they lose energy and should therefore get dangerously close to the nucleus right up until they crash into it!  Such an atom
would not therefore be stable.


                BOHRS ATOM

       In order to take account of atomic stability, in 1913 Niels Bohr created a new model of the atom:  The orbits of the electrons can't be just anywhere but are "quantified"; only certain particular orbits are permitted for the electron. It's not until one jumps from one orbit to another that it can emit (or absorb) light.  The picture below illustrates the Niels Bohr model.


This site was made by a student, and was made possible by a Science teacher, and the service provider, zoominternet.

The pictures and the information on this site were taken from the websites: http://perso.club-internet.fr/molaire1/e_histoire.html
                                                                                      www.aip.org/history/electron/jjhome.htm

Additonal information was taken from the World Book Encyclopedia 1973.

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