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Atomic Structure
The Discovery of ...
... the neutron.
... the missing mass

Following the discoveries of Has Geiger, Ernest Marsden and the explanation of Ernest Rutherford, it was clear that an atom had at least two parts; a very dense positively charged center where the protons were located, and a much lighter volume of space surrounding this center where the negatively charged electrons were to be found. But this still left two problems...

  • Where and how were the electrons arranged, (and how did this contribute to the physical and chemical properties of the atom or element)?
  • Not all the mass of an atom could be accounted for by the positively charged protons. Where and what was missing?

Hydrogen was easy. The hydrogen atom was known to contain one proton (mass = 1, charge = +1), which was located at the atomic center (sometimes called the 'nucleus of the atom'), and one electron (mass = 0.0002, charge = -1), which seemed to be somewhere outside the center, but taking up a clearly defined volume of space.

It was fairly easy to remove this electron from a hydrogen atom (and put it somewhere else). This left a single, positively charged subatomic particle that behaved as a hydrogen ion, or a proton, depending on how it was treated.

Atoms of the element Helium had different properties. It was known that helium atoms had two protons in their dense centers and had two electrons somewhere in the surrounding volume of space. But these electrons were much harder to remove and put somewhere else. If sufficient force were used to strip away both electrons, then the remaining dense center behaved exactly like an alpha particle

mass is missing

But this was where the first problem arose. The number of units of positive charge (+2) found in the dense center of the helium atom could not account for the total mass of the helium atoms. This mass was known from experiments in which alpha particles were deflected in their paths by large magnetic fields. It had a value of four units of mass, not two units! Some mass was missing.

This problem got worse and worse as the atoms got bigger and bigger. The dense center of an atom of uranium, for example, has a massive mass of 238 units, but only +92 positive charges.

It was certainly possible that there were 238 protons in the center of an atom of uranium, and along with them there were 146 electrons. Since the electrons had very little mass, they would not change the weight of the dense center, but their negative electrical charges would neutralize 146 protons, leaving a net charge of +92, which was what was observed. It sort of made sense, and was thought to be the case for about 10 years.

Then a better explanation came along.

not whole numbers

Thanks to the work of a several scientists, it was becoming clear that the atomic mass (weight) of an element was not a pure whole number as envisioned by Dalton and others. Chlorine, for example, apparently had an atomic mass of 35.457 units, barium 137.34, boron 10.811 and cadmium 112.40. This was fact one.

It was also becoming clear that there were different "forms" of an element. William Crookes, in England, separated uranium into at least two different forms, one of which was very radioactive, much more radioactive that the original uranium sample.


After a lot of work and thinking Frederick Soddy proposed, in 1913, that an raw element actually consisted of various atomic versions, for which he coined the term isotope. Isotopes of an element differ from one another by their atomic mass or weight. Ordinary thorium, for example has a mass of 232, but, as Soddy said, there is another version with a mass of 228. Lead has several versions, including lead 210, lead 214, lead 212 and lead 211.

All of these isotopes could be separated from one another, as shown by Francis William Aston when he and J.J. Thomson fired neon atoms though a very strong magnetic field and were able to show that the lighter isotope had a more curving path than the heavier isotope. One had a mass of 20 units and the other a mass of 22 units.

fractional atomic masses

Suddenly there was the reason for fractional atomic masses! Raw neon was made up of a mixture of neon 20 and neon 22 in the ratio of about 10:1 [Note: it is now known that there is third isotope of neon (21) present in the ratio 1:400]. The observed atomic mass of neon (20.183) actually represents the average mass or weight of the mixture of all the isotopes.

In all isotopes of all the elements, however, the number of positive charges (i.e. protons) appeared to remain the same, this was the atomic number and seemed to determine the chemical properties of the element. This was fact two.

Something was going on in that dark, dense, positively charged center of all atoms, and it was time to break a few apart and look at the pieces.

making protons

Rutherford was one of the first to start knocking the atomic center about. He arranged for a stream of alpha particles (mass=4, charge=+2) to be stopped by a thin metal screen that absorbed them all. None were allowed to penetrate the screen and hit the zinc sulfide scintillation detector behind it. No flashes of light were seen, so no particles were getting through.

dislodging protons

Then he introduced a small amount of hydrogen into the chamber and into the path of the streaming alpha particles. Suddenly the scintillation detector lit up with a burst of new light flashes that were different from those seen when alpha particles hit the zinc sulfide. What was going on?

The explanation that Rutherford devised was that a rapidly moving alpha particle hit a hydrogen molecule head on. The force of this collision was hard enough to dislodge a proton from the hydrogen molecule and send it flying through the metal screen and into the zinc sulfide, causing a scintillating flash of light.

When this experiment was repeated, but using nitrogen gas instead of hydrogen gas, he also saw scintillations and the only explanation he could think of was that the alpha particle, when it struck the center of an atom of nitrogen, broke is apart and sent a fast moving proton in the direction of the detector. It looked as if the centers of atoms could be disrupted and their contents dispersed. But how to detect the pieces?

seeing tracks

A Scottish physicist, Charles Thomson Rees Wilson had the answer. He had devised a glass container that was attached to a very strong vacuum piston (pump). When the chamber was filled with air saturated with water vapor, and the piston rapidly pulled outwards, the pressure in the chamber suddenly drops, the air and water expand, cools dramatically, and takes on a very unstable, supersaturated state.

As Rutherford showed, a alpha particle passing through this supersaturated "cloud" ionizes the atoms it finds in its path and leaves a train if tiny, tiny water droplets behind it. These "tracks" though the clouds can be seen, photographed and studied for what they can say about the particles leaving them in their wake.

Using the Wilson Cloud Chamber and a strong magnetic field, it was possible to see the tracts left by a number of the subatomic particles. Charged particles (electrons, protons, alpha particles, etc.) would be bent in different directions depending on their charges, and by different amounts depending on their masses. See below...

Tracks left by electrons (light beta particles) are faint and often jiggle around as the tiny electron is easily knocked off course by almost anything (including other electrons!). A proton is heavier but of the opposite charge so it bends less, but in the opposite direction. A massive alpha particle makes almost a straight line to the other side of the chamber, and if it hits anything the track takes on a sudden bend, and it will vanish if it picks up two electrons and becomes a neutral helium atoms once again. So, only charged particles leave trails. This fact was to become an important clue during the discovery of the last subatomic particle.

attack the center

Now the attack on the dense center of an atom could begin in earnest. Rutherford bombarded then center of nitrogen atoms with his alpha particles, but, although the results were very interesting, nothing more was learnt about the mysterious mass.

In 1930 Walther Bothe and H. Becker used their alpha particles to attack the center of a beryllium atom and reported that a strange new radiation had been seen which had great penetrating power. Two years later the daughter of Marie Curie, Irene Joliot-Curie, used this radiation from a beryllium atom to bombard paraffin, a hydrocarbon and found that it would knock protons out of the atoms found there.

James Chadwick took the final step. He began from the supposition that this new radiation consisted of particles (like all the other kinds) that had a mass. To find this mass he bombarded boron atoms with them and then looked to see what had happened to the mass of the boron. From his calculations, it appeared that this new particle had about the same mass as a proton, but when he tried to see one in the Wilson Cloud Chamber - nothing happened!

no charge

From this he deduced that, just as the neutral helium atom left no trail (see above) this new particle must be neutral as well and not carry any positive or negative charges. A completely new particle had been discovered. He adopted a name for this particle that had already been coined - neutron, and he was awarded the Nobel Prize for it in 1935.

This new particle solved all the outstanding problems of atomic structure. Werner Heisenberg in Germany did all the calculations and pronounced that the atomic model that contained protons and neutrons in its center (the 'nucleus') was much more satisfactory than the tentative one in which protons and electrons were found together in the 'nucleus'. Also, it explained everything from isotopes (different number of neutrons), to power of the atomic number to explain the chemical properties of an element.

All that was left to discover and explain was the placing and properties of the electrons that occupy much of the volume of an atom.

© 2003, Professor John Blamire