PERIODIC TABLE

 
 
 

HISTORY OF THE PERIODIC TABLE

Jerome Garcia

A graduate of University of South Florida with B.A. in chemistry. and  curently teaching at Glenn Hills High School.  Mr. Garcia teaches Physical Science and Chemistry.

EARLY PERIODIC TABLE

In the early nineteenth century, scientists began to seek ways to classify the elements. In 1817,  Johann Doebereiner,a German chemist, attemped to classify the elements.  Dobereiner found that the properties of the metals (Calcium, Barium, and Strontium) were very similar.  He also noted that the atomic mass of strontium was about midway between those of calcium and barium.  He grouped these three elements into what he termed the triad.  Doebereiner found several groups of three elements with similar properties.  In each case, he found that the second element has an atomic mass that was about halfway between those of the first and the third elements in the triad.
In 1863 John Newlands, an English scientist suggested another classification.  He arranged the elements in order of their increasing atomic masses.  He noted that there appeared to be a correlation between increasing atomic masses and the repeition of similar properties in every eighth element.  He arranged all the elements known at that time (49) into groups of seven containing seven elements per group.  Because the same properties repeated every eigt elements, he  referred to the arrangement as the law of octaves.  .
Dimitri Mendeleev, a Russian, chemist, proposed a similar idea.  He suggested, as Newlands, that the properties of the elements were a function of their atomic masses.  Mendeleev believed that similar properties occurred after periods (horizontal rows) that could vary in length although he placed seven elements each in his first two periods, he placed seventeen elements each in the next two rows.  Mendeleev left some blank spots in order to group all the elements with similar properties in the same column.  Mendeleev predicted the properties and atomic masses of several elements that unknown at the time.  Mendeleev state that the properties of the elements are a periodic function of their atomic masses.  This is called the periodic law.
 

MODERN PERIODIC TABLE


Henry Moseley X-ray experiment in 1913 showed that the nucleus of each element has a positive charge (atomic number).  As a result of Mosseley's work the periodic table was revised.   It now has as its basis the atomic number rather than its atomic mass.  The modern statement of the periodic law is the propetties of the elements are a function of their atomic numbers.  The atomic number of an element indicates the number portons in the nucleus of each atom of the element.

Links Page

Periodic Propetties

Lesson Plan


http://rpsec.usca.sc.edu//Classwork/731sp2001/Lesson/Garcia/ garcia2.html (January, 2001)
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