Question

What were the limitations of Dobereiner’s Classification?

Answer 1

Hint: In 1829 Johann Wolfgang Dobereiner discovered the existence of families of elements with similar chemical properties. The atomic weight of Na is almost the average of the atomic weights of Li and K. The elements of triad have almost similar properties.

Complete step by step answer:
Let us see what is Dobereiner’s law of triads?
Dobereiner 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 law states that, when elements are placed in 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.

Let us try to understand better with the help of an example!
In the alkali metal group, consider elements lithium (Li), sodium (Na) and potassium (K). All these elements are metals, they are highly reactive. From the Dobereiner’s law of triads, the atomic mass of the middle element, in this case Sodium, should be the arithmetic mean of Lithium and Potassium.

Now, let us see what were the Limitations of Dobereiner’s classifications?
All known elements could not be classified into groups of triads on the basis of their properties.
Not all groups obeyed the Law of Triads. For example, the Nitrogen family does not obey the Law of Triads.
Doberiner could identify only three triads. He was not able to prepare triads of all known elements.
For very low mass or for very high mass elements, the law was not holding good. Take F, Cl, Br.

Note: The only advantage of Dobereiner's research was that it made chemists look at elements in terms of groups of elements with similar chemical and physical properties. This eventually led to rigorous classification of elements and the modern periodic table of elements, as we now know it, was discovered.