Periodic Law of Mendeleev

Periodic Law of Mendeleev:

Newlands concept of arrangement of elements was further developed to a large extent by Mendeleev (1869), a Russian chemist. Mendeleev pointed out that when the elements are arranged in the order of their increasing atomic weights, those which are chemically similar recur at regular intervals. This relation between the elements was termed the Periodic Law, which he stated as given below-

The properties of elements are periodic functions of their atomic weights.

On the basis of the periodic law, Mendeleev arranged the elements in the form of a table, known as Mendeleev’s Periodic Table.

In order to lay more stress on similarities in properties of elements rather than rigidly following the increasing order of their atomic weights, Mendeleev had to deviate from the periodic law. While framing his table, he kept some vacant places and predicted the existence of yet unknown elements to fill up these gaps. He could even foretell the properties of these elements by studying the properties of neighbouring elements. Such elements were discovered later and were found to have almost the same properties as had been predicted by Mendeleev.

The latest form of Mendeleev periodic table also includes those elements which were not known when the original table was framed. In this table:

• The vertical columns are known as groups. There are nine groups in all, numbered from 0 (zero) to VIII. Zero group consists of inert gases which were discovered after Mendeleev had published his periodic table. The elements in any given group are of very similar chemical behaviour. The nine groups in the periodic table are regarded as-
  1. Seven regular groups (Group I to Group VII)- Normal groups.
  2. One transitional group (Group VIII)- A group of transitional elements.
  3. One zero group of inert gases.
• The groups I to VII are subdivided into two subgroups A and B. Group 0 is either placed before the group I or it is made the sub-group B of group VIII.
• The horizontal rows are known as periods. These are seven in number. The number of elements present in each period is shown in the following table.

The 14 are rare earth elements beginning after Lanthanum (Atomic Number 57) are known as Lanthanides. These elements resemble in properties. The elements, which begin after Actinium (Atomic Number 89) in the 7th period are referred to as Actinides.

Mendeleev‘s periodic table proved itself of greatest value regarding the advancement in the study of chemical elements. It had, however, certain anomalies which could not be explained. Some of the anomalies are-

• Isotopes of the elements having different atomic weights could not be provided separate places in the periodic table. If we attempt to arrange them according to their atomic weights in the normal manner, the periodic character of the table would be upset.
• As exact atomic weights became known, it was found that some elements with higher atomic weights preceded those with lower atomic weights. For example- Argon (Atomic Weight = 39.94, Group 0) comes before Potassium (Atomic Weight = 39.10, Group I); Tellurium (Atomic Weight = 127.6, Group VI) comes before Iodine (Atomic Weight = 126.9, Group VII).

It appeared that it was not the atomic weight but some other fundamental property of the elements which could give a better understanding of the periodicity in the properties of the elements. This property was found to be the atomic number.

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