10th Class Science Periodic Classification of Elements Mendeleevs Classification

       Mendeleeu's   Classification

In 1869, a Russian chemist, Mendeleev, on the basis of physical and chemical properties discovered a relation known as "PERIODIC LAW". "The properties of element are the periodic function of their atomic masses".

Mendeleev arranged the known elements according to increasing order of their atomic masses. According to him, the fundamental property of an element was atomic mass. He arranged these elements into eight groups and then further divided into subgroups.

             Merits of Mendeleev’s Periodic Table

Mendeleev's periodic table offered the following advantages in understanding the properties of elements.

• There was a regular gradation in physical and chemical properties of elements.
• The group number of an element indicates highest oxidation state that it can attain.

• There were many vacant spaces in Mendeleev's periodic table for the elements to be discovered. He named them Eka-Boron, Eka-Aluminium and Eka-silicone He also predicted the properties of these undiscovered elements including atomic masses. These elements were discovered as Sc, Ga and Ge with same features as he predicted.
• Mendeleev's arrangement helped to correct atomic masses of a number of elements.

            Demerits in Mendleev's Periodic Table

• For placing the elements in proper groups, the order of the elements according to atomic mass was reversed in certain cases. He placed Iodine (127) after Tellurium (128) Potassium (39) and Mi (58) after Co (59), Which is against his periodic law/but correct according to properties.
• Mendeleev's periodic table does not provide a clear idea about the structure of atom.
• Lanthanide and Actinide have been assigned places in the periodic table, which is against the periodic law.
• Alkali metal and coinage metals (Cu, Ag and Au), which differ widely in properties, are placed into the same group.
• There is no separate position for isotopes in his periodic table.
• The change in atomic mass of two successive elements is not constant.

Hence it is not possible to predict the number of missing elements by knowing the atomic masses of two known elements.

         According to Newland's how many elements are there in nature?

(a) 20

(b) 25

(c) 30

(d) 56                                   

(e) None of these

Answer: (d)

          What was the basis of Mendeleev's classification?

(a) Atomic Mass

(b) Atomic Number

(c) Valency

(d) Isotopes                                      

(e) None of these

Answer: (a)

          In which year Mendeleev's formulated the law of classification?

(a) 1865

(b) 1870         

(c) 1869

(d) 1875                                               

(e) None of these

Answer: (C)