Question

can u explain dobereiner's traids with the help of an example except that of Li, Na and K...
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Answer 1

Here's your answer in the pic

Answer 2

There are three triads

Li l Ca l Cl

Na l Sr l Br

K lBa l I

In all these triads the average of the sum of the mass of the first and last element is approximately equal to the mass is second element(middle most) .

You can refer Google to find the masses of elements and verify it.