CBSE Class 12 Chemistry Notes: Periodic Properties – Mendeleev’s Periodic Law

Mendeleev’s Periodic Law:

Mendeleev, a Russian Chemist used broader range to physical and chemical properties to classify the elements. The physical and chemical properties of elements are periodic functions of their atomic weights. If the elements are arranged in the order of their increasing atomic weights, after a regular interval elements with similar properties are repeated (inert gases were not discovered till then)

MENDELEEV’S PERIODIC TABLE
Mendeleev arranged the discovered 63 elements in the periodic table into 7 horizontal rows known as periods and 8 vertical columns known as groups numbered 1 to 8.

Mendeleev Suggested:

• He excluded certain elements and assigned them a separate independent position.
• Leaving gaps for the then undiscovered elements
• When the properties of elements did not correspond to what is expected of the group they were named by prefixing Eka to the preceding element e.g. Eka boron (Silicon); Eka silicon (Germanium), Eka aluminum (Gallium); Eka Maganese (Technitium)

Uses of Mendeleev’s Periodic Table

• Systematic study of the elements
• Atomic weights of elements were determined with the help of periodic table. Atomic weight = Valency  Equivalent weight = Group number x Equivalent weight.
• Atomic weight of elements were corrected. Atomic weight of Be was calculated to be 3  4.5 = 13.5 by considering its valency 3. Mendeleev calculated it 2 * 4.5 = 9.
• Discovery of new elements – In Mendeleev’s periodic table two consecutive members differs by two or three units in the atomic weight. Where this gap was more, the gaps were left in the periodic table.

Defects of Mendeleev’s Periodic Table:

• Position of hydrogen is uncertain. It has been placed in IA and VIIA groups because of its resemblance with both the groups.
• No separate positions are given to isotopes.
• It is not clear to which group lanthanides and actinides belong to.
• Although there is no resemblance except valency of subgroups A and B, they have been put in the same group.
• Order of increasing atomic weights is not strictly followed in the arrangement of elements in the periodic table. For e.g. Co (At.wt. 58.9) is placed before Ni (58.7) and Ar (39.9) is placed before K (39)

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