Science and Technology for UPSC Examinations [Chemistry] Part 7- Basics of Periodic Classification

Periodic Classification:

The arrangement in which the substances of the same properties reappear again at regular interval based upon some fundamental properties is called Periodic classification or Periodical arrangement.

• This classification attempts to arrange the chemical elements to arrange them on the basis of fundamental properties.
• The practice started in 19th century. In the middle of the 19th century, Dmitri Mendeleev a Russian Physicist propounded the Mendeleev law. This law states:
• The physical and chemical properties of the elements are periodic functions of their atomic weights.
  

This law states that if the elements are arranged in the order of increasing atomic weights then after certain definite number there would be the elements which would have almost same properties. The elements known by the times of Dmitri Mendeleev were arranged by him in a table and it is called Mendeleev table.

Salient Features of Mendeleev’s Law

The salient features of Mendeleev’s law were:

• The elements, if arranged according to their atomic weights, exhibit an apparent periodicity of properties.
• Elements which are similar as regards to their chemical properties have atomic weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which increase regularly (e.g., K, Rb, Cs).
• The arrangement of the elements, or of groups of elements in the order of their atomic weights, corresponds to their valencies, as well as, to some extent, to their distinctive chemical properties.
• The elements which are the most widely diffused have small atomic weights.
• The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body.
• We must expect the discovery of many yet unknown elements–for example, elements analogous to aluminum and silicon–whose atomic weight would be between 65 and 75.
• The atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous elements.
• Certain characteristic properties of elements can be foretold from their atomic weights.

Striking features of Mendeleev’s Table

• The first striking feature of the Mendeleev’s periodic table is that it predicted discovery of new elements and kept black space for them.
• He predicted the discovery of Ekaboron which later identified with scandium, Ekaaluminium which later identified with gallium, Ekamanganese which later identified with technetium and Ekasilicon which later identified with germanium.
• He also predicted about an element between thorium and uranium in 1871 and it was identified as protactinium.
• Another striking feature was variance from atomic weight order. However, in modern times the elements are arranged in atomic number order.

Shortcomings of Mendeleev’s Table

• The drawback of Mendeleev’s table was that it did not include any of the noble gases, because the noble gases were not discovered by that time.
• Another shortcoming was that there was no place for isotopes. The position of Hydrogen remained disputed. Some elements with same properties were kept in different groups.

The atomic number was just a sequential number till the beginning of the 20th century. This sequential number was based upon the element’s atomic weight. It was in 1914, that Henry Moseley discovered a relationship between an element’s X-ray wavelength and its atomic number. Based upon the nuclear charge, he re-sequenced the table which was called modern periodic table. For the first time, Henry Moseley was successful in showing that atomic numbers had an experimentally measurable basis. Some elements were re-sequenced despite of the correct order of their atomic weight in the Mendeleev’s atomic table. Thus atomic number got it prime place in the periodic table with the efforts of Henry Moseley.

• The Moseley’s periodic law states that
• Physical and Chemical properties of the elements are the periodic function of their atomic numbers.
• This removed almost all drawbacks in Mendeleev’s periodic table.