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Q.1. Which of the following is not a Dobereiner triad ?
(a) Cl, Br, I
(b) Ca, Sr, Ba
(c) Li, Na, K
(d) Fe, Co, Ni

Dear Student,
first we have to know what is doberenier law of triad

According to doberenier all element occured in group of three when arranged in increasing order of  atomic masses .He referred it as triads.
In triad elements have same chemical properties.he said that the mass of middle element of a triad is arithmetic mean of atomic masses of other two elements .    

for e.g.
in triad of Lithium,Sodium,Potassium

Element      symbol       Atomic Number Lithium            Li                        7Sodium           Na                      23Potassium      K                        39
the average mass of Li and K gives average mass equal to Na.

answer is (D)
because average atomic mass  of iron and nickel is not equal to atomic mass of cobalt.

Regards

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