Periodic Classification of Elements

1. Overview: There are 118 elements are known at present. All have different properties. Out of them, only 94 are naturally occurring. As there is a chance that different elements may be discovered in future and scientist tries to gather more and more properties of elements. It is required to organise all elements in a well-defined manner so that one can obtain maximum information at once. Earlier, there was no way to organise or tabulate them altogether in informatics way. Hence, classification of elements in proper and scientific way was needed.
   
2. Early attempts at the classification of elements: The earliest attempt to classify elements resulted in grouping them into two parts as Metals & Non-Metals. Later, further classifications were also tried out. Some of the attempted ways of classification are given below:
   
3. Dobereiner’s triads: In 1817, Johann Wolfgang Dobereiner, a German chemist, tried to arrange the elements with similar properties into groups. He identified these groups having three elements each. Accordingly, he named these groups ‘Triads’.
   
4. He showed that when the three elements in a group triad were written in the order of increasing atomic masses, the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements. The same can be understand by the following tabulation.
   

1. This Triads is not successful to most of the elements discovered at that time. Hence, this way of classification was not found to be useful.
   
2. Newlands’ Law of Octaves: In the year 1866, an English scientist named John Newlands tried to arranged the then known elements in the order of increasing atomic masses. He started with the element having the lowest atomic mass (Hydrogen) & ended with the element having the last element. He found that every eight elements among them having similar properties to that of the first so arranged. He named this arrangement as ‘Octave’ in the tune of musical notes.
   
3. Later, it was found that the Law of Octaves was applicable only upto Calcium (Ca). After that, every eight elements did not possess similar property to that of the first of the Octaves. Further, he assumed only 56 existed and no more elements would be discovered in future. In order to fit elements into his table, he adjusted two elements in the same slot, and in this manner he also put two unlike elements under the same note. Hence, Octaves arrangement was not successful.
   
4. 
   Questions (Page: 81)
   
   1. Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
      

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   2. What were the limitations of Debereiner’s classification?
      

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   3. What were the limitations of Newlands’ Law of Octaves?
      

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1. Mendeleev’s Periodic Table: With the concept of Newlands’ law of Octaves, further scientists begun to find way to classify the elements on the basis of Atomic Mass. A Russian chemist named Dmitri Ivanovich Mendeleev, gave out his classification of element principle. His principle is:
   

    

   “The physical & Chemical properties of an element is periodic function of their Atomic Mass”.
   

    

When Mendeleev started his work, only 63 elements were known. He examined the physical and chemical properties of the elements on the basis of their Atomic Mass. He arranged the elements in ascending order of their atomic mass and constructed a table of elements which is known as periodic table. Mendeleev’s periodic table consisted of 8 vertical columns called groups & 7 horizontal rows called period. However, a new group called group ‘O’ was added to this period table after the discovery of inert gases.

5.2 While developing the periodic table, there were a few instances where

Mendeleev had to place an element having slightly greater atomic mass before

an element with a slightly lower atomic mass. The sequence was introduced so

that elements with similar properties could be grouped together. Further, Mendeleev left some gapes in his periodic table, instead of looking upon these gaps as defects, Mendeleev boldly predicted the existence of some elements that were not discovered at that time. It was success of Mendeleev prediction that led chemists not only to accept his periodic table but also recognise him as the originator of the concept on which it is based. Noble gases (inert gages) like Helium (He), Neon (Ne) & Argon (Ar) have been mentioned in may a context before this. These gases were discovered very late because they are very inert and present in extremely low concentrations in our atmosphere. The strength of Mendeleev’s periodic table was that when inert gases were discovered they could be placed in a new group without disturbing the existing order so arranged.

2.9     Limitations of Mendeleev’s Periodic classification:

(a) He could not assign a correct position of hydrogen in his periodic table, as the properties of hydrogen resembles both with alkali metals as well as with halogens.

(b) The isotopes of the same element will be given different position if atomic number is taken as basis, which will disturb the symmetry of the periodic table.

(c) The atomic masses do not increase in a regular manner in going from one elements to the next.

As a result, it was not possible to predict how many elements could be discovered between two elements.

Questions (Page: 85)