Periodic Classification of Elements Essay

898 Words Jan 13th, 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 arranged 56 elements in his table. Table 2: Newlands Octaves
He compared his table to the octaves of music. It is for this reason that he called his model the Law of Octaves or Newlands Law of Octaves.
• Limitations of Newlands
This law was not applicable throughout the arrangement. It was applicable only till calcium.
• Newlands assumed that only 56 elements would exist in nature and believed that no more elements would be discovered. However, several elements were discovered in the following years. These elements did not follow the Law of Octaves.
• The positions of cobalt and nickel could not be explained according to Newlands Law of Octaves. He kept cobalt and nickel in the same slot. They were also placed in the same column as fluorine and chlorine, which have completely different properties.
• The properties of iron are similar to those of cobalt and nickel. However, iron was placed away from them in a different column
Mendeleev’s Periodic Law:
This law states that the properties of elements are a periodic function of their atomic masses. This means that if elements are arranged in the increasing order of their atomic masses, then their properties will be repeated after regular intervals.
Only 63 elements were known when Mendeleev first started his work. He arranged these elements in the form of a table in the increasing order of their atomic masses. This table is known as Mendeleev’s periodic table as shown in the table. His

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