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Answer to Question #4629 in Inorganic Chemistry for Brooke

Question #4629
What are the symbols for one of the triads observed by Dobereiner?
Expert's answer
Dobereiner’s triads

Dobereiner in 1817 observed that certain elements, which had similar chemical properties, could be grouped together. When these elements were arranged in increasing order of their atomic masses, they generally occurred in groups of three. These groups were called triads. He noticed that the atomic mass of the middle element of the triad was the arithmetic mean of the other two elements of the triad. This was known as the Dobereiner’s law of triads. The law states that : when elements are placed in order of the ascending order of atomic masses, groups of three elements having similar properties are obtained. The atomic mass of the middle element of the triad is equal to the mean of the atomic masses of the other two elements of the triad.

Examples of Dobereiner Triads :

Examples of triads

In the alkali metal group, consider elements lithium (Li), sodium (Na) and potassium (K). All these elements are metals, they are highly reactive and they show valency of +1. The Dobereiner’s triad for alkali metal group can be shown as:
Elements Symbol A (atomic mass)
Lithium Li 7
Sodium Na 23
Potassium K 39

Consider another group of elements : sulphur (S), selenium (Se) and tellurium (Te). All these elements are non-metals, tending to show metallic behavior. When you arrange them in the ascending order of their atomic masses, they obey Dobereiner’s law. See the table given below.
Elements Symbol A (atomic mass)
Sulphur S 32
Selenium Se 79
Tellurium Te 128

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