Mendeleev's Periodic Table

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In 1869, Mendeleev classified the then known 56 elements on the basis of their physical and chemical properties by giving a law known as the periodic law. The name periodic law stems from the fact that the properties of the elements orderly recur in a cyclic fashion. His periodic law states that "the physical and chemical properties of the elements are periodic function of their atomic masses". This means that when the elements are arranged in the order of their increasing atomic masses, the elements with similar properties recur at regular intervals. Based on this law all the known elements were arranged in the form of a table known as periodic table. Elements with similar properties recur at regular intervals and fall in certain groups or families. The elements in each group were similar to each other in many properties.

Merits of Mendeleev's periodic table

Mendeleev's periodic table was one of the greatest achievements in Chemistry with some of its important contributions as follows:

• As the arrangements are classified into groups, it was very useful in studying and remembering the properties of a large number of elements in a systematic way.

• This classification helped in correcting the atomic masses of some of the elements like gold, beryllium and platinum based on their positions in the periodic table.

• Mendeleev could predict the properties of some undiscovered elements like scandium, gallium and germanium. By this intuition, he had left gaps for the undiscovered elements while arranging elements in his periodic table.

Demerits of Mendeleev's periodic table

In spite of the above advantages, Mendeleev's periodic table suffered defects as follows:

• The position of hydrogen was not correctly defined. It was placed in group I although it resembles both the group I elements - the alkali metals and the group VII elements-the halogens, in their properties.

• In some cases Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses. Thus, the position of these elements was not justified e.g. cobalt (atomic mass 58.9) was placed before nickel (atomic mass 58.6).

• Isotopes were not given separate places in the periodic table although Mendeleev's classification is based on the atomic masses.

• Some similar elements were grouped separately while some dissimilar elements were grouped together. For example copper and mercury are similar in their properties but were placed separately. Copper was placed in group I although it did not resemble the elements of this group.

• Mendeleev could not explain the cause of periodicity in the elements.

• The position for lanthanides and actinides were not included in this table.