Early Ideas of Atomic Structure

As mentioned in the previous section titled A Short History of Chemistry, many scientists identified elements, determined their characteristics, similarities, and differences, and designed symbols for them. Using unique experiments, scientists devised ways to define the structure of atoms and determine atomic weights, sizes, and electrical charges as well as energy levels for atoms. 

Many of these men and women recognized the existence of some order in the manner in which chemicals relate and react to each other. Although these scientists could not see the atoms themselves, they were aware that the structure of each element s atoms has something to do with these characteristics. There were several attempts to organize the elements into a chart that reflected the particular nature of the atoms for these elements. Before the periodic table of the chemical elements was developed as we know it today, several relationships had to be established. (See the next section for more on the periodic table of the chemical elements.) 

Two of Rutherford s students conducted a classic experiment to determine the structure of an atom. They beamed alpha particles through a sheet of gold foil that was 1/50,000 of an inch thick. Since the thickness of this thin foil was only about two thousand atoms of gold, it 

Wolfgang Pauh (1900-1958), an American physicist, was awarded a Nobel Prize in 1945 for developing the exclusion principle. In essence, it states that a particular electron in an atom has only one of fom energy states and that all other electrons are excluded from this electron s energy level or orbital. In other words, no two electrons may occupy the same state of energy (or position in an orbit around the nucleus). This led to the concept that only a certain number of electrons can occupy the same shell or orbit. In addition, the wave properties of electrons are measmed in quantum amounts and are related to the physical and, thus, the chemical properties of atoms. These concepts enable scientists to precisely define important physical properties of the atoms of different elements and to more accmately place elements in the periodic table. 

The terms shell, orbital, and energy level are sometimes used interchangeably. In this text we use the term shells most often because it is descriptive and conveys the image of three-dimensional layers or structures of electrons smrounding the nucleus. The term 

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