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Atomic Structure
The Discovery of ...
... Electrons
Discovery of Electrons

In the 1800s electricity was new, exciting and the subject of a lot of study. The English chemist Humphrey Davy (1778-1829) had built the world's largest battery (over 250 metallic plates) and pushed the very high currents this battery could generate through all kinds of solutions, compounds and substances in the hope that the high energies involved would pull apart the chemical constituents. Using this technique, he discovered many things, including new elements, and aroused the interest of his pupil - Michael Faraday (1792 - 1867).

Faraday coined many of the terms still used today, including electrolysis, electrolyte, electrodes, anode, anions, cathode and cations. He also got the idea to pass an electrical current (discharge) through a complete vacuum, just to see what happened - if anything.

Unfortunately his methods of producing an appropriate vacuum were not good enough and he never really succeeded, but a German glass blower - Heinrich Geissler - certainly did. His apparatus consisted of a glass tube in which an anode (the positive pole, or plate) was at one end, and the cathode (the negative pole, or plate) was at the other end. His superior vacuum pump removed all the air from the tube, and he connected the anode and the cathode to the appropriate ends of a powerful battery.

At high enough voltages electricity certainly seemed to be able to leap across the vacuum between the oppositely charged plates, but that was not all. On the wall opposite to the negative cathode, the glass glowed a strange, greenish color.

Discovery of Cathode Rays

By 1876 Eugen Goldstein was certain that the glow was being caused by a new kind of radiation that started at the negative plate and radiated across the vacuum until it hit the glass. Since Faraday had called the negative pole or plate the cathode, Goldstein called his radiation, cathode rays.

What could they be?

German researchers were convinced that the cathode rays traveled in waves, like light, whereas the British researchers thought that the rays were composed of tiny, tiny charged particles. This difference in concept caused a lot of rivalry at the time and, although none of them were ever to know it, they were both right - sort of!

William Crookes (an English physicist), among several others, including Julius Plucker, showed that bringing a magnet next to the sides of the tube caused the cathode rays to bend in a way that strongly suggested that they were made up of electrically charged particles - not waves. This observation allowed Arthur Schuster (in 1890) to calculate the charge-to-mass ratio of these "particles" from the amount of bending he observed in a cathode ray tube when it was magnetized.

Joseph John Thomson

All these interesting discoveries attracted the attention of the English physicist Joseph John Thomson ("J.J." - to his friends). By 1894 Thomson was able to announce that these rays traveled much more slowly than rays of light (he found a value of 1.9 x 107 cm/sec. Light travels at 3.0 x 1010 cm/sec.). So it was more likely that they were particles than waves.

Thomson also carried out a series of experiments using a better designed cathode ray tube that incorporated two small plates, between which the rays had to travel. By connecting these plates to the opposite poles of a battery, an electric field was generated at right angles to the path of the rays. This had a dramatic effect. Just as had happened with a magnetic field, so with the electric field, the rays were bent!

By April 30th, 1897 J.J. Thomson was ready with his big announcement; the cathode rays consisted of negatively charged particles (which he called "corpuscles") that were only less than 1/1000th of the mass of a hydrogen atom. This meant that they could not be simply charged atoms (ions), or any then known particle. This was something very new.

The electron

Thomson was awarded the Nobel Prize in 1906 for his "discovery" of the first sub-atomic particle; the electron. This discovery strongly implied that Dalton was wrong and that the atom was not the smallest particle of matter. It looked as if the atom could be broken down into even smaller pieces, and to Thomson these smaller pieces were his negatively charged electrons.

Very rarely in science is a new discovery the result of a single person's work or ideas, and the idea that cathode rays were interesting, possibly particles, traveled in straight lines, carried energy and had other intriguing properties were the conclusion of many other researchers, including the "positivist" Walter Kaufmann, who actually had many of his results before those of Thomson. However, Thomson gets the historical credit for "discovering" the electron because he had the courage to insist that his corpuscle was one of the building blocks of matter (and atoms).

No one else had this courage.

In 1904 he was bold enough to try and build the first model of what an atom might look like. He had one major problem; all electrons were negatively charged, without some way of neutralizing these charges there was no way of keeping thousands of electrons together, in one place, so they could become part of an atom.

Thomson proposed this solution:

... suppose that the atom consists of a number of corpuscles moving about in a sphere of uniform positive electrification...

This is often called Thomson's "plum pudding model", or more likely these days the "chocolate chip cookie" model in which the dough of the cookie is a cloud of "uniform positive electrification" and the chocolate chips are the small, negatively charged electrons.

As a model of atomic structure it had a very short life, but it got people thinking, and one of those who thought more than others was Ernest Rutherford, whose work took the search to the next level.

© 2003, Professor John Blamire