Lesson 3-4

The Periodic Table

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The Origin of the Periodic Table

     When a person is confronted with a large number of items, it is only natural to look for similarities that can be used to develop a classification scheme.   A person who collects baseball cards may group his cards according to team or position.  Biologists classify all living organisms in a five-kingdom classification system, based on similar characteristics.  Early Chemists studied the group of known elements, and tried to come up with logical classification systems based on what they knew.

     An early attempt was made by a German Chemist named Johann Dobereiner, in 1817.  Dobereiner noticed that there were several groups of three elements which shared certain properties.   For example; chlorine, bromine and iodine all share antiseptic properties.   Dobereiner called each group of three elements with similar properties a "triad".  He also noticed that when he arranged the elements of a triad by atomic mass, the middle element had an atomic mass that fell close to the middle of the other two atomic masses.

     Almost fifty years later, an English Chemist named John Newlands proposed an updated classification system.   Newlands had noticed that when the 49 known elements were arranged in order of increasing atomic masses, certain properties would repeat every eighth element.  He arranged the elements into seven groups of seven elements and called his system the "law of octaves."

     In the 1860's a German Chemist named Lothar Meyer was developing a periodic table based on the idea that when the elements were arranged by atomic mass, certain properties were repeated periodically.

     In 1869 Dimitri Mendeleev, a Russian Chemist, published the first periodic table.  It had eight columns, and it contained blank spaces for elements that Mendeleev predicted must exist, although they had not yet been discovered.  His predictions turned out to be correct.  New elements were discovered, and they fit into the spaces that he had left in his periodic table.    

     It was not until 1914 that Henry Moseley corrected the periodic law, based on his discovery of the atomic numbers of several elements.  This modern periodic law states, "The periodic properties exhibited by the elements are a function of the atomic numbers."

The Structure of the Periodic Table

    The Periodic table holds so much more information than most people realize.  The average person can see that the table shows the atomic number and atomic mass of each element, but a student of Chemistry learns that there is much more there, for someone who knows how to read the table correctly.  In this lesson, you will learn more about the structure of the periodic table, so that you will be able to extract some more of this additional information.

     Unlike Mendeleev's table, the modern Periodic Table is arranged according to atomic number.  Remember that it is the atomic number, or nuclear charge, that determines the identity of the element.  The horizontal rows of elements on the periodic table are called periods.  The vertical columns are called groups or families.  By looking at the column that an element is found in, you can predict the valence shell configuration with a good deal of accuracy.  By looking at the period that the element is found in, you can determine the energy level which the valence shell is found.  Look at the examples shown on the table below;

Table 3-4a

     Sodium (Na)  is in the third period (row), so its valence shell is in the third energy level.   Furthermore, it is in the first column of the s section, so its valence configuration is 3s¹. 

     Zirconium (Zr) is in the fifth period, so its valence shell is in the fifth energy level.  It is in the second column of the d section, but the d electrons are never in the valence shell.   Zirconium has a valence configuration of 5s², because 5s² is filled before its last electron (4d²) is filled.  There are a few exceptions, but most of the transition metals will have two valence electrons.

     Oxygen (O) is found in the second period, and in the fourth column of the p section.  Because the p is filled after the s electrons, oxygen's valence shell would be 2s² 2p⁴.

Metals, Nonmetals and Metalloids

     The elements of the periodic table belong in three basic categories; metals, nonmetals and semimetals (or metalloids).   We will discuss the criteria that Chemists use to classify the elements in the next lesson, but for now you should be able to locate the areas on the periodic table below;

Table 3-4b

    As you can see, the vast majority of the elements are considered metals.  Some of the elements that are considered metals would not surprise you.  For example, the average person would think of copper, gold, and iron as metals, and indeed, chemists do also.    However, some of the elements that are considered metals by Chemists may surprise you.  Did you know that chemists consider calcium and potassium metals?   Sodium does not look at all like what most people think of as metallic, it is soft and white, yet a Chemist considers it a metal.  Below you will see a table that summarizes the distinguishing characteristics of metals, nonmetals and metalloids.

*  Remember that these are general characteristics, there are exceptions.

Elemental Families and Groups     

     Elements that are found in the same group (column) tend to have similar properties, because they have similar valence shell configurations.  This is not as true of the elements that are found in columns that contain metals, nonmetals and metalloids, like columns 14 and 15.  Some of the groups contain elements that are similar enough to be given a family name.  The family names that you should know are shown on the table below;

Table 3-4d

Please forward all questions, comments and criticisms to Gregory L. Curran.
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Last Modified February 07, 2008