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Question

(a) The modern periodic table has been evolved through the early attempts of Dobereiner, Newland and Mendeleev. List one advantage and one limitation of all three attempts.

(b) Name the scientist who first of all showed that atomic number of an element is a more fundamental property than its atomic mass.

(c) State Modern Periodic law.

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Solution

(a) (i) Dobereiner :

Advantage : Group three elements are having similar properties and they were called as the Dobereiner triads. He arranged them in order of increasing atomic mass and the mass of middle element was equal to the average of the other two elements.
Limitation : He was able to identify only 4 traids but he failed in his attempt to group nitrogen, phosphorus and arsenic because atomic mass of phosphorus is not the average of other two.

(ii) Newland :

Advantages : He arranged the elements in horizontal rows in order of increase in atomic mass. The property of every 8th element was similar to the 1st element.

Limitation : His arrangement was only applicable to lighter elements.

(iii) Mendeleev :

Advantage : He arranged the elements in the order of there increasing atomic mass and he corrected the mass of beryllium from 4.5 to 13.5 u.

Limitation : Position of hydrogen and isotopes were not justified in his periodic table.

(b) Henry Moseley showed that atomic number of an element is a more fundamental property than its atomic mass.

(c) Modern periodic law states that "The properties of elements are the periodic function of their atomic numbers. Repetition of properties is due to the same outer electronic configuration."

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