The Atom

Essay by ktvHigh School, 12th gradeA, January 1996

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A good discussion of the pros and cons of using the atomic bomb to end World War II -

AP Physics

Period 2

The Atom

In the spring of 1897 J.J. Thomson demonstrated that the beam of glowing matter in

a cathode-ray tube was not made of light waves, as 'the almost unanimous opinion of

German physicists' held. Rather, cathode rays were negatively charged particles boiling

off the negative cathode and attracted to the positive anode. These particles could be

deflected by an electric field and bent into curved paths by a magnetic field. They were

much lighter than hydrogen atoms and were identical 'what ever the gas through which the

discharge passes' if gas was introduced into the tube. Since they were lighter than the

lightest known kind of matter and identical regardless of the kind of matter they were born

from, it followed that they must be some basic constituent part of matter, and if they were a

part, then there must be a whole.

The real, physical electron implied a real, physical atom:

the particulate theory of matter was therefore justified for the first time convincingly by

physical experiment. They sang success at the annual Cavendish dinner.

Armed with the electron, and knowing from other experiment that what was left

when electrons were stripped away from an atom was much more massive remainder that

was positively charged, Thomson went on in the next decade to develop a model of the

atom that came to be called the 'plum pudding' model. The Thomson atom, 'a number of

negatively electrified corpuscles enclosed in a sphere of uniform positive electrification'

like raisins in a pudding, was a hybrid: particulate electrons and diffuse remainder. It

served the useful purpose of demonstrating mathematically that electrons could be arranged

in a stable configurations...