Periodic Classification of Elements

884 Words Jan 22nd, 2011 4 Pages
PERIODIC CLASSIFICATION OF ELEMENTS
Introduction
There are 115 elements that are known at present. Some elements have similar properties whereas some others have completely contrasting properties
Scientists began to look for some pattern in the properties of these elements

Dobereiner’s triads
In 1817, Johann Wolfgang Dobereiner, a German chemist, classified elements into groups based on their properties. He kept all elements having similar properties in one group. Most of his groups had three elements each. Thus, he called these groups as triads
He also gave a law known as the Law of Triads.

It states that when three elements in a triad are listed in the increasing order of their atomic masses, the atomic mass of the middle
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He had predicted that some elements were yet to be discovered. He left these gaps deliberately so that these undiscovered elements could get a place in his periodic table.
Mendeleev named the undiscovered elements using the Sanskrit word Eka (meaning one) as a prefix, with the name of the preceding element in the same group. For example, gallium was not discovered in Mendeleev’s time. Therefore, he left a gap for it in his periodic table and named it Eka-aluminium. He also predicted the properties of these undiscovered elements based on their positions in the periodic tab . It can be clearly observed from the table that Mendeleev’s predictions were almost exact. This extraordinary achievement made Mendeleev’s periodic table very popular.
• Noble gases were not discovered at the time when Mendeleev gave the periodic table. These were discovered in recent times as they are very inert and are present in very low concentrations in the atmosphere. When these gases were finally discovered, they got a place in his periodic table as a separate column. The accommodation of these gases in the periodic table did not disturb the positions of other elements. This underlined the strength of Mendeleev’s periodic table
Limitations of Mendeleev’s periodic table:
1. Hydrogen’s position was not justified in Mendeleev’s periodic table.
He positioned hydrogen in the first column above alkali metals. He did so
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