What is Mendeleef’s periodic law and mention its essential features ?

When Mendeleef arranged the elements in order of increasing atomic masses, he observed that the elements with similar properties appeared at regular intervals. This led him to state his famous periodic law in 1869. The Mendeleef’s Periodic Law states that:

“Properties of the elements are a periodic function of their atomic masses.”

While working only this classification of elements he wrote,

“- When I arrange the elements according to the magnitude of their atomic masses beginning with the smallest, it becomes evident that there exists a kind of periodicity in their properties . I designate by the name “Periodic Law”, the mutual relation between the properties of the elements and their atomic masses. These relations are applicable to all the elements and have the nature of a periodic function.”

MENDELEEF’S PERIODIC TABLE

When Mendeleef arranged elements in the order of increasing atomic masses, he obtained a chart consisting of vertical columns and horizontal rows. He called this chart Periodic Table.

He called this chart Periodic Table.

While doing so, he laid more stress only the similarity only the properties of the elements. The order of increasing atomic masses was not rigidly followed. Thus, when an element did not seem to fit into the expected position, he left a gap for it. He predicted that new elements would fill those gaps as and when discovered.

Thus, he had the vision to leave gaps for the undiscovered elements. He could even predict the properties of such undiscovered elements.

Essential features of the Mendeleef’s periodic table:

Mendeleef’s periodic table had the following features

(i) The vertical columns were called groups, and the horizontal rows are called the periods.

(ii) The elements in each group resembled with each other in many properties. All elements of a group show the same valence, which is equal to the group number, 8-group number.

(iii) The properties of elements changed gradually in any period while going from left to right.

(iv) There were in all eight great groups (Group I to Group VIII), and seven periods. To accommodate more elements the periods 4, 5, 6 and 7 were divided into two halves. The first half of the elements were placed in the upper left corner, and the second half in the lower right corner of each box.

Although, Mendeleef’s peroidc table was a brilliant attempt for the classification of elements, but it had certain drawbacks in it.

Inversion in the periodic table:

In Mendeleef’s table, certain pairs of elements were placed in the reverse order of atomic masses. For example, cobalt (Co) having higher atomic mass had been placed before nickel (Ni) having lowers atomic mass. Also, argon (40) was placed before potassium (39). This contradicted his periodic law.

Position of isotopes: isotopes have similar chemical properties, but different atomic masses. According to Mendeleef’s periodic law, therefore isotopes of an element must be given separate places in the periodic table.

Grouping of chemically dissimilar elements:

Certain chemically dissimilar elements have been grouped together in Mendeleef’s table. For example, Cu and Ag have no resemblance with alkali metals (Li, Na, K, etc.), but these have been grouped together in Group I.

Separation of chemically similar elements:

Certain elements which appear to be chemically similar (like Cu and Hg, Au and Pt etc) have been placed in separate groups. Position of Hydrogen: Hydrogen forms both positive ions like alkali metals and negative ion like halogens. Thus, hydrogen resembles both. Therefore it can be placed both in Group I and Group VII. In Mendeleef’s table, the position of hydrogen was not made clear. From the above anomalies, it became clear that the atomic mass does not appear to be sound basis for the periodic classification of elements.