HISTORY OF THE PERIODIC TABLE
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.
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