Study the data of the following three categories A. B and C.
Answer
Category 
Name of the element and Atomic Mass 
Checking triad 
A 

Mean of first and last = (7 + 39)/2 = 23 Atomic mass of middle = 23 Since they are the same , it is a dobereiner’s triad. 
B 

Mean of first and last = (14 + 74)/2 = 44 Atomic mass of middle = 31 Since they are not the same , it is not a dobereiner’s triad. 
C 

Mean of first and last = (10.8 + 69.7)/2 = 40.25 Atomic mass of middle = 27 Since they are not the same , it is not a dobereiner’s triad. 
Therefore, category 3 is a dobereiner’s triad.
Why did Mendeleev place elements of category A, B and C in three different groups?
 Mendeleev arranged the elements according to
 increasing atomic masses
 The elements having similar chemical properties were placed together
 The elements of category A, B and C are similar among themselves ( have similar chemical properties ) but each category is different from each other.
Therefore, category A,B and C were placed in different groups.
Is Newland law of octaves applicable to all the three categories ? Give reason to justify your answer.
 In newlands law of octave, the first and eighth elements show similar properties .
 This rule is not valid for elements after Ca in the periodic table. Therefore, the newland’s law of octave is not applicable for all the three categories.
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