What is the periodic classification of elements? And why do we need to classify them?
All of us must have visited a library sometime. How are books organised in a library? Books are arranged by author name or by subject thus helping us to retrieve the books quickly. Similarly, the aim of the periodic classification of elements is to simplify the study of elements.
There are 118 elements in nature. Of these, 98 are naturally occurring. Elements are being discovered with each passing day. Therefore, scientists have been trying to classify the elements based on shared properties shared by the elements. One of the earliest methods was to classify the elements as metal or non-metal.
In 1817, Dobereiner a German chemist made the first attempt at periodic classification of elements. He grouped elements based on some properties, and he identified some groups that contained three elements each. He called these groups the Dobereiner?s Triad. He noted that when the elements are arranged in the ascending order of their atomic weight, the element in the middle had an atomic weight that was the average of the other two elements.
Dobereiner identified only three triads from the elements known at that time. Therefore, this system of periodic classification of elements did not become famous.
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Newlands, an English chemist, arranged the elements according to their atomic weight. The lightest element was hydrogen, and the heaviest element was thorium. He noted that every eight elements in the table had similar properties. He called this as the law of octaves (after the octaves in music).
However, there were problems associated with the law of octaves. The law worked fine for lighter elements up to calcium. Beyond calcium, every 8th element did not possess the same properties. Also, with the discovery of new elements, the law of octaves was found to be inadequate for the periodic classification of elements.
In 1872, Mendeleev, a Russian chemist published his periodic classification of elements. He classified the 63 known elements into groups and periods. Elements were organised based on their atomic mass. On the basis of his work he postulated the periodic law- ?the properties of the elements are the periodic function of their atomic mass?.
There were some gaps in Mendeleev?s periodic classification of elements. He predicted that these holes corresponded to elements that were undiscovered at that time.
Although Mendeleev?s periodic classification of elements was an important step in chemistry, there were a few limitations to his periodic classification of elements. The position of Hydrogen in Mendeleev?s table was ambiguous. Secondly, isotopes were a challenge to Mendeleev?s periodic classification of elements. Thus, there was a need to refine Mendeleev?s periodic classification of elements to incorporate the newly discovered elements and also to plug the loopholes in Mendeleev?s table. ?We will learn about the modern periodic table in another article.
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