Question

Why H, He and Li are not considered as triad?
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Answer 1

Answer :-

H(Hydrogen), He( helium) and Li(lithium) are not considered as Triad because their properties are much different.

Difference in their properties:

Hydrogen is much reactive gas and is a non metal,on the other hand Lithium is an alkali metal and also Helium is an inert gas which doesn't react with others. These all have contradictory properties so they are not placed in triad.

Learn More !!

What is a triad?

Elements with similar properties were arranged into groups of three elements called Triad.

Characteristics of Triad?

It showed that when three elements were in a triad written in order of increasing atomic masses, the atomic mass of middle element was roughly average to the sum of other 2 elements.

Why are several elements with similar properties not placed in triad?

There are many elements which have similar properties but are not placed in triad because many elements were not discovered at the time of Dobereiner so, he couldn't make triad for them. Noble gases which are inert in nature were discovered very later. So, Dobereiner could not place them in its triad.

Limitations of triad?

Dobereiner could identify only 3 such triad i.e. only 9 elements. He was not able to prepare triad for all the known elements at that time.

Which elements are included in Dobereiner's triad?

$\begin{array}{l|l|l}\sf Triad\; 1&\sf Triad\; 2&\sf Triad \;3 \\ \\ \rm Lithium&\rm Calcium& \rm Chlorine\\\rm Sodium&\rm Strontium &\rm Bromine\\\rm Potassium&\rm Barium&\rm Iodine \end{array}$