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Much of the work in this chapter (and in the electrons chapter) owes its development to one woman, Marie Curie. Through her pioneering work, she was able to purify and concentrate the radioactive components of uranium ores. During 1898, she and her husband worked to concentrate the radioactivity and isolate new elements that she believed were in the ore and causing a variable radioactivity signal. In July of that year the element polonium was named after their home country of Poland. In December of the same year, a second element radium was isolated. These radioactive materials could then be converted into particle "guns" capable of delivering fine focussed beams of radioactive material.

Rutherford quickly ceased on these particle guns and used them to extreme effectiveness. Beginning in 1906 he bombarded metal foils such as gold with speeding alpha particles (positively charged helium nuclei). He noticed that most simple passed through as he believed they might. However, some were scattered by large angles and on occasion deflected back towards the particle gun itself.

Since the foils were generally about two atoms thick and most particles passed directly through he concluded that the majority of the atom must be empty space, but what about the huge deflections? For such a deflection to occur, he concluded, something massive must be within the atom that is not only massive but positively charged. From this he evolved his theory of the nuclear atom. He decided that the center of the atom contained a very tiny nucleus which is positively charged and contains all the protons (and as later discovered by Chadwick, the neutrons also) of the atom. It had to be small since most particles passed straight through but it must also contain virtually all of the atoms mass. surrounding this nucleus and taking up the remaining space of the atom, must be the electrons.

This gave a whole new realm to thought on the nature of chemical reactions. Now it became clear that during a chemical reaction, only the outermost electrons would be affected. This quickly explained how ions formed but left one question. "How does the nuclear atom of one element differ from that of another?"

The answer to this was provided in part by the works of German physicist Max Theodore Felix von Laue (1879-1960), English physicist Charles Glover Barkla (1877-1944) and English physicist Henry Gwyn-Jeffreys Moseley (1887-1915) who used and studied x-rays to obtain various information about atomic nature. The most easily understood of these is the work of Moseley, who in 1913 found that the wavelength of x-rays decreased (energy increased) smoothly with increasing atomic weight of the elements emitting them. This inverse relationship as Moseley argued, was due to the size of the positive charge. The larger the charge, the shorter the wavelength (higher the energy) of the characteristic x-rays.

More impressive was his ability to calculate the positive charge on each nucleus from the wavelength of the x-rays. It was determined that hydrogen was +1, helium was +2, lithium was +3, and so on up to +92 for uranium.

The size of the nuclear charge assigned to the name of atomic number and suddenly it became clear that when Mendeléev had arranged his elements in order of what was considered atomic weight was really atomic number. Additionally, Mendeléev had arranged his periodic table by considering the valence of the different elements, rather than their electronic arrangements, which were unknown to him. It became clear to several scientists that this valence was governed by the electron arrangement.

Much of the early work in this field was due to German chemist Richard Abegg (1869-1910) who, earlier, pointed out in 1904, that the inert gases must have a particularly stable configuration of electrons since they had no tendency to gain or lose them. This work was prior to that of Moseley and so it was in 1916 that the true idea of electron configuration and its significance began to was fully explained by American chemists Gilbert Newton Lewis (1874-1946) and Irving Langmuir (1881-1957) who, independently extended the notion suggested by Abegg that atoms may gain or lose electrons to obtain a configuration similar to that of the inert gases, to molecules where electrons are not gained or lost. They proposed that each atom could contribute an electron to a shared pool that remained in the outermost electron shell of both atoms.


Designed & maintained by Paul Charlesworth, Chemistry Department, Michigan Tech. April 07, 1999.