In Chemistry, Dobereiner triads are defined as, any of several sets of three chemically same elements, the atomic weight of one, which is nearly equal to the mean of the atomic weights of the other two elements. Such triads, including the calcium-strontium-barium, sulfur-selenium-tellurium, and chlorine-bromine-iodine, were noted by the German chemist named J.W. Döbereiner between the years 1817 and 1829. This triad was the earliest atomic-weight classification of these elements.
Dobereiner triads are groups of elements with identical properties that were discovered by Johann Wolfgang Dobereiner, a German chemist. He also observed that the groups of three elements (so-called triads) could be formed, where all the elements shared the same physical properties and chemical properties.
Dobereiner's law of triads also stated that the atomic masses of the first and third elements in the triad would be approximately equal to the atomic mass of the second element in the triad. Also, he suggested that this particular law could be extended for the other quantifiable properties of elements such as density.
The alkaline earth metals strontium, barium, and calcium made up Dobereiner's first triads, which were discovered in 1817. The other three more triads were identified by 1829, where these triads are tabulated below:
This triad was made of the alkali metals sodium, potassium, and lithium.
The arithmetic mean of the masses of lithium and potassium is 23.02, which is almost identical to sodium's atomic mass.
As explained before, barium, strontium, and calcium formed another one of the Dobereiner triads. Let us see in the below table:
The mean of the masses of calcium and barium corresponds to a value of 88.7.
The halogens bromine, iodine, and chlorine constituted one of the triads.
The mean value of the atomic masses of the iodine and chlorine elements is given as 81.1.
The elements selenium, tellurium, and sulfur made up the fourth triad.
The arithmetic mean of the masses of both the first and third elements in this triad corresponds to the value of 79.85.
Cobalt, nickel, and Iron constituted the last of the Dobereiner’s triads.
However, the mean of atomic masses of both nickel and Iron corresponds to a value of 57.3.
About Dobereiner Classification of Triads
Let us look at the Dobereiner classification of triads in detail.
Johann Wolfgang Dobereiner, a German chemist, made an effort. It is thought to be one of the first attempts to categorize elements into categories.
The earliest classification has categorized elements into both metals and nonmetals. It was not easy to classify some of the elements, such as boron, which exhibited the properties of both metals and non-metals as well. After some further research, a German scientist named Dobereiner arrived at a hypothesis in 1829.
He has found that when the elements are arranged into the groups of three in the same order of their increasing atomic mass, then the atomic mass of the element, which comes into the middle, is said to be the arithmetic mean of the rest of the two. Based on this manner, he arranged three elements in one group, which is called 'Triad.' This particular arrangement of elements is referred to as Dobereiner's Triads.
Limitations of Dobereiner's Triads
Let us see the key shortcomings of Dobereiner's method of classifying the elements in the below list:
Achievements of Dobereiner's Triads
Johann Döbereiner, who is a German chemist, has developed the law of triads, which states that "If the elements having similar chemical properties are arranged in the groups of three, the atomic weight of the middle element will be considered almost equal to the arithmetic mean of the two terminal element's sum." He has discovered many triads between 1817 and 1829. On, three of them are given as examples in the table:
The achievements of this idea can be given as: it has given an idea that the elements can be arranged in specific ways and their properties can be predicted, and it has become the precursor to the other periodic tables such as John Newlands' "law of octaves" and also others including Dmitry Mendeleev's "Periodic Table" and Lothar Meyer's "Periodic Table of Elements."
Advantages of Dobereiner's Attempts to the Modern Periodic Table
In the year 1896 Dobereiner has stated that if we would arrange the elements based on the increasing atomic mass, then there is a repetition of properties after every 8 elements. But as we know that, isotopes contain varied atomic mass. So, as per the Dobereiner concept, they should contain varied properties and also take more space.
And now, many various isotopes take place, but all have the same properties to the same elements. So, many attempts have continued to solve this particular problem. After that, in 1913, Moseley gave the concept of atomic number, which means, if the elements are arranged based on the atomic numbers, we need not write isotopes, and it saves the periodic table to remain long. So, Dobereiner also has contributed to the modern periodic table.