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Posted by on Feb 14, 2011 in Physical Chemistry | 0 comments

Discovery of the electron

In 1897 J.J. Thomson discovered the electron while doing experiments at the Cavendish Laboratory at Cambridge University.  At that time the fact that Nature was made up of atoms and molecules was not fully accepted.  Scientists still believed in the ether, which the Michelson-Morley experiments finally disproved, also in 1887.  So J. J. Thomson’s experiment not only confirmed the existence of the most important elementary particle: the electron, he also showed that atoms are not the indestructible building blocks of matter that the Greeks had suggested, but parts of atoms and molecules

Although J.J. Thomson did what good scientists do, he characterized the particles in the Cathode ray tube, he could only get the charge to mass ratio, e/m.  Was the mass very small or was the charge very big?  No one knew.  So Thomson first suggested that electrons were uniformly distributed throughout an atom.  This Plum Pudding model was incorrect.

In a later ingenious experiment, Millikan was able to find the electron charge and from the charge to mass ratio.  This meant the mass of the electron was extremely small, 9.11 x 10-31 kg.  Now by small we mean that it is about 1800 time smaller than the smallest atom, the H atom.  Hence Thomson had discovered a new particle.  After this, Ernest Rutherford at McGill University did scattering experiments and found that the nucleus is tiny, and massive, and most of an atom is a cloud of electrons.  We had our first description of the structure of atoms.  Let us look at Thomson’s experiment in a bit more detail.

Science advances as new experimental techniques were discovered.  Earlier glass blowing techniques had evolved so it was possible to embed metal in glass, to act as electrodes, and to evacuate the air from the tube.  This cathode ray tube was a popular exhibit at science shows since the tube glowed.  They were the first “Neon Light”, although different gases were used than neon.

Here are the parts of a cathode ray tube.  Although we know that electrons move from the cathode to the anode, at that time it was not known if the particles were negative or positive.  A beam passed through a hole and left marks on the detector screen.  These positions could be deflected from the center by electric and magnetic fields.  But charged particles also moved towards the anode, so must be positive.

If the cathode ray was completely evacuated then no glow could be seen.  However if some air, like oxygen, nitrogen, or a noble gas was used, then these gases could fluoresce. Here is an atom, and atoms have different energy depending on their state, and these energies are quantized.

The electron hits an atom; is absorbed; and jumps to an excited state.  This is unstable and the energy cascades down and is emitted as light at different wavelengths so the tube glows.

Thomson was able to build on the work of others Even though he could only get the charge to mass ratio, he knew he had discovered something significant.  He said.

“We have in the cathode rays matter in a new state.“

Let us look as some results.  It was first thought that Positive ions (cations) are formed and move to the cathode – called “canal waves”, but these turned out to be the charged atoms, not the cathode rays.  However the observed beam was negatively charged; moved in the opposite direction; and did not have the properties of positively charged atoms.

Here charged condenser plates are added to the CRT and the beam can be deflected up towards the positive plates. It can only be concluded that the beam is composed of negatively charged particles.  Similar experiments with magnetic fields showed that the beam also had magnetic properties.  By measuring these deflections, and knowing the energy needed, Thomson was able to calculate the acceleration of the particles.

So the force can be written in terms of the unknown mass. Now electrical force is just charge times field strength, so this must be the charge on the particle, the unknown electron charge, e, and the applied field strength, which is known, capital E.  Dividing by the mass and combining with the experimental values, this ratio is -1.78 times ten to the 8th Coulombs per gram.  But this ratio does not give us the actual charge nor the actual mass, only the ratio.

Let us hear what Thomson had to say about the smallness of the electron:

“Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen? –which itself is so small that a crowd of these atoms equal in number to the population of the whole world would be too small to have been detected by any means then known to science.”

A common consequence of new discoveries is the development of new technologies follow.  The discovering of the electron opened up many doors on the road to quantum mechanics.  Here is one consequence of charged ions: the TV tube: Thomson could not have envisioned television, but his discovery was a step in that direction.

Mass spectroscopy is also based upon the charge to mass ratio: Pass charged particles through an electric field and depending on its mass and velocity, the particles experience different deflections.  This allows an accurate separation of particles based upon their mass, and is the basis for the important analytical technique called Mass Spectrometry.  Of course for this, we need to know the actual electron charge, and not just the charge to mass ration.

This was resolved by the famous Millikan Oil Drop experiment which I will discuss next.

Find similar topics and their explanations, along with  interactive multimedia animations, in the Physical Chemistry e-book by Laidler, Meiser, Sanctuary

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