Friday, May 7, 2010

Divisible Part 4

The Invisible Man

The neutron was discovered very late on in this story as it has no charge so was very difficult at the time to observe. In 1920 Rutherford predicted the existence of a neutral particle of similar mass to the proton. He came to this conclusion because he knew that the alpha particle was just a helium ion which he knew to have two electrons and two protons. With two protons and two electrons the mass did not add up so he assumed that a chargeless particle was responsible for the missing mass. He was not the only person to reach this conclusion with American nuclear chemist William Harkins (1873-1951) even going so far as to name this invisible particle "neutron". In the early 1930’s German physicists Bothe and Becker were studying the effects of high energy alpha particles bombarding light elements. It was noted that an unusual radiation was being produced. It was very hard to observe any meaningful data from this result and thus proved difficult to formulate any concrete conclusion regarding this phenomenon.

This phenomenon was again noted in 1932 by two French physicists Irene Joliot-Curie (1897-1956) and Frederic Joliot (1900-1958). They bombarded hydrogen containing compounds and found that protons were ejected at high speed. They had assumed that they had noted a form of gamma or x-rays; however they did not realize the significance of this discovery.

It was not until James Chadwick (1891-1974) in the same year devised an experiment to eliminate the idea of gamma or x-rays. In his experiments he bombarded boron gas with alpha particles. The particles emitted from these collisions were allowed to collide with paraffin wax. Protons were knocked out of the paraffin. From this observation he concluded that the protons were in collision with some massive particle. From the energies involved he concluded that the new particle must be of similar mass to the proton and also have no charge as it made no track in the cloud chamber. Chadwick won the Nobel Prize for this discovery in 1935. With the discovery of the neutron several physicists believed we had now found the key to the physics within the nucleus. Werner Heisenberg (1901-1976) published several important papers on the subject in which he concluded that the nucleus was comprised of protons and neutrons held together by some unseen force.



The discovery of the neutron would solve many of the problems noted with atomic masses. It was known since the early 20th century that not all atoms of the same element were the same mass. Thompson observed this in several experiments on neon gas; in these experiments several tracks for neon were recorded. This should not have been the case if all neon atoms were the same. From this observation Thompson concluded that not all atoms of neon were identical and that they differed in terms of their mass. This apparent oddity could now be accounted for by changing the number of neutrons present in the atom.

Nuclear Models

Several models of the atom were proposed throughout the 19th century until the model we arrive at now in mid 1930’s. In the 19th century an idea was theorized by Dalton that the atom was a solid sphere but this has the obvious short coming of being unable to explain charge and electricity. This problem was tackled by the Thompson who theorized the ‘plum pudding’ model in which electrons were present in a mass of material whose charge was positive. This at least explained why atoms were electrically neutral.

The evidence for the existence of atoms themselves has a long history stretching back to the age of the Romans. In c. 60 BC a Roman poet records observations of dust particles in the air. In this poem he noted the apparent random motion of the dust. The author hinted that there may be some unseen particles so small we can’t see them and these are what are responsible for the motion. This motion is what we refer to as Brownian motion named in honour of Robert Brown (1773-1858). Brown was a botanist whose observations of pollen suspended on water gave rise to this phenomenon. He observed that the pollen grains appear to jostle about in an apparent random motion. He repeated this experiment with dust to eliminate any suggestion that the pollen was somehow alive, but could not give a satisfactory answer for this motion. It would take a brilliant young physicist to link this motion in pollen grains to the atom. In 1905 Albert Einstein (1879-1955) published a paper of his observations. In it he discussed the motion of the pollen was linked to the vibrations at the atomic level. This, he offered, was direct evidence for the existence of atoms and molecules.



The inner structure of the atom remained elusive until Rutherford and his scattering experiment. In this experiment he established that atoms were comprised of mostly empty space with a tightly packed positive nucleus at their core. In Rutherford’s model the electrons orbited the nucleus like planets orbiting the sun. This wasn’t without its problems as the model could not explain why the electrons do not collapse into the core. Niels Bohr (1885-1962) further expanded on this model in which the electrons could only have certain allowable orbits.



In Bohr’s model electrons could only orbit within certain energy levels. This prevented the electrons from losing energy (rotating bodies radiate energy) and falling into the centre as they were restricted to a minimum energy. From this model the atomic spectrum of hydrogen was explained. However this model could not be extended to explain other elements as there spectra were far more complex.

With the discovery of the neutron in the early 1930’s it did appear then that the full picture of the atom was reviled. We now had a nucleus with neutrons and protons which in turn was orbited by electrons. Physics now had an explanation for the atomic structure and why we have isotopes. Also from this model radioactivity could be explained in a manner which resulted in charges within the atom either in the emission of energy from electrons changing orbits, electrons decaying from the atom, or alpha particles ejected from the nucleus.

There were still several problems with this model which could not be explained. At this time physicists had no explanation for why the protons could be in such close proximity in the nucleus. From classical physics they should be repelled from each other and the core of the atom should fly apart. But with the discovery of the neutron it hinted that this may be partly responsible for holding the nucleus together.  The atomic age brought about the end of classical physics and opened up the new age of quantum physics. 

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