Mendeleev periodic table

 Mendeleev's Periodic Table

Introduction:

Dmitri Ivanovich Mendeleev, a Russian chemist, in 1869, delivered a remarkable contribution to arrange chemical elements in the Periodic table. The Mendeleev periodic law was modified with the following statement.

      "The physical and chemical  properties of the elements are the periodic functions of their atomic masses".

Description of Mendeleev's Periodic Table:

a) In the periodic table, the elements are arranged in vertical columns called groups and horizontal rows are known as periods.

b) There are seven periods or horizontal rows in Mendeleev's periodic table. In order to accommodate more elements the periods 4, 5, 6 and 7 are divided into two halves. (As shown in above figure, the first half of the elements are placed in the upper left corners and the second half of them occupy the lower right corners in each box).

c) There are eight groups (noble gas was not introduced in Mendeleev's time which was introduced later as zero group, giving the total of nine groups) designated by the Roman numbers from I to VIII. Group VIII contains traits of elements. Later, in modified form of periodic table, each group except VIII has been divided into two sub-groups.

d) Zero-group: Mendeleev arranged only about 60 elements known at that time. After the discovery of the inert gas elements by William Ramsay and Lord Rayleigh, these were included in the zero group. Ramsay and Rayleigh were awarded Nobel prize in 1904 for their discoveries.

Advantages of Mendeleev's Periodic Table;

The formulation of periodic law by Mendeleev revolutionized the systematic study of chemistry. The scientific benefits of the periodic table are listed below:

- Mendeleev periodic table proved very useful in simplifying the study of new elements and their compounds.

-Systematic and scientific study of the elements.

-Discovery of new element.

-Correction of doubtful atomic weight.

Defects of Mendeleev's Periodic Table:

In spite of its great usefulness, the periodic table suffered from certain serious defects and anomalies. The major shortcomings of the tables are :

Position of hydrogen

-A correct position for hydrogen could not be assigned in the periodic table. Hydrogen could be placed in the alkali metals group (group IA) as well as in the halogens group (group VIIA). It is still not certain whether to place H-atom in group IA or VIIA.

Anomalous positions of some elements

-Although the elements on Mendeleev's periodic table had been arranged in order of increasing atomic masses, in some cases, the element with higher atomic mass preceded the element with lower atomic mass. Some examples are-

I) Argon (39.9) is placed before potassium (39.1).

II) Cobalt (59.9) is placed before nickel (58.6).

Position of isotopes

-There was no place for isotopes of various elements, which were discovered later. If we arrange the elements in order of their increasing atomic masses, then, it becomes difficult to assign the proper place for a large number of isotopes. For example, hydrogen has 3 isotopes but is assigned only one position.

Grouping of elements

-Some similar elements were separated and dissimilar elements were grouped together. For instance, the pairs such as copper and mercury, silver and thallium, barium and lead have similar properties but these were placed in different groups. On the other hand, elements of group IA such as lithium, sodium, and potassium were grouped with dissimilar elements such as copper, silver, and gold.

Position of lanthanides and actinide

-Fourteen elements that follow lanthanum are called lanthanides and fourteen elements that follow actinium are called actinides. These were not given proper places in Mendeleev's periodic table.

Position of metals and non-metals

-No attempt had been made to place metals and non-metals separately in the periodic table. Most of the groups were mixed groups consisting of metals, nonmetals, and metalloids.

Cause of periodicity

-No proper explanation has been offered for the fact why the elements placed in a group show resemblance in their properties.

Position for noble gases

-Original Mendeleev's periodic table did not indicate any of the noble gases, which were discovered later. These were added by Sir Willian Ramsay as Group Zero, without the disturbance to the basic concept of the periodic table.