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  • Identify the group which is not a Dobereiner triad:Option: 1 Li, Na, KOption: 2 Be, Mg, Cr<

Identify the group which is not a Dobereiner triad:

Option: 1

Li, Na, K


Option: 2

Be, Mg, Cr


Option: 3

Ca, Sr, Ba


Option: 4

Cl, Br, I


Answers (1)

best_answer
We know that according to Dobereiner's law of triads the atomic mass of the middle element of a triad is the arithmetic mean of the atomic masses of the other two elements. 

The group which does not follow Dobereiner’s law of triad is Be, Mg, Cr.

It is because Cr is not of the same group as Be and Mg are. Be and Mg belong to group second whereas Cr belongs to group sixth.

Moreover, the arithmetic mean of the atomic masses of the other two elements is not equal to the atomic mass of the middle element.

Atomic mass of Be = 8, Mg = 24, Cr = 51.9

Arithmetic mean = 51.9+82=29.9.  This arithmetic mean is not equal to atomic mass of central element.

 Therefore, Option(2) is correct

Posted by

vinayak

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