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What is Triad Chemistry?

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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

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:

Triad 1

This triad was made of the alkali metals sodium, potassium, and lithium.

Triad

Atomic Masses

Sodium

22.99

Lithium

6.94

Potassium

39.1


The arithmetic mean of the masses of lithium and potassium is 23.02, which is almost identical to sodium's atomic mass.

Triad 2

As explained before, barium, strontium, and calcium formed another one of the Dobereiner triads. Let us see in the below table:

Triad

Atomic Masses

Strontium

87.6

Barium

137.3

Calcium

40.1


The mean of the masses of calcium and barium corresponds to a value of 88.7.

Triad 3

The halogens bromine, iodine, and chlorine constituted one of the triads.

Triad

Atomic Masses

Bromine

79.9

Chloride

35.4

Iodine

126.9


The mean value of the atomic masses of the iodine and chlorine elements is given as 81.1.


Triad 4

The elements selenium, tellurium, and sulfur made up the fourth triad.

Triad

Atomic Masses

Selenium

78.9

Tellurium

127.6

Sulfur

32.1


The arithmetic mean of the masses of both the first and third elements in this triad corresponds to the value of 79.85.


Triad 5

Cobalt, nickel, and Iron constituted the last of the Dobereiner’s triads.

Triad

Atomic Masses

Cobalt

58.9

Nickel

58.7

Iron

55.8


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:

  • This model was made obsolete by the discovery of new elements.

  • The triads did not suit newly discovered elements.

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:

First

Triad

Element

Lithium

Sodium

Potassium

Arithmetic Mean

Atomic

mass

7.0

23.0

39.0

(7 + 39)/2 = 23.0

Second

Triad

Element

Calcium

Strontium

Barium

Arithmetic Mean

Atomic

Mass

40.0

87.6

137

(40 + 137)/2 = 88.5

Third

Triad

Element

Chlorine

Bromine

Iodine

Arithmetic Mean

Atomic

Mass

35.5

80.0

127.0

(35.5 + 127)/2 = 81.5


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.

FAQ (Frequently Asked Questions)

1. Why was the Dobereiner's Triad Discarded?

Answer: Doberniers triad was discarded because the elements in the triads did not contain similar physical or chemical properties. Also, it did not hold true for all the elements and resulted in a disordered element collection.

2. Give the Limitations of Dobereiner's Triads?

Answer: Let us look at the limitations of Dobereiner's triads.

It is said that only the sum of 5 Dobereiner's triads was identified; even many known elements did not fit into any of the triads. Owing to these specific shortcomings, other classifying element methods were also developed.

3. Are Nitrogen, Carbon, and Oxygen Triads?

Answer: Actually, the chemist - Dobereiner has found that a few groups of three elements that contain the same properties, for example, sodium, potassium, and lithium, the atomic mass (or the mass number) for the second element was given as the average of both the first and third element. A similar element was found for some other groups of three elements, for example, nitrogen (14), carbon (12), and oxygen (16). These particular groups became Dobereiner's Triads.


Hence C, N, and O2 are considered as the Dobereiner's triads.

4. Explain Why Nitrogen and Carbon Do Not Separate in Normal Air?

Answer: Because the molecular weight does not enter into it, density matters, for example, dodecane contains a higher molecular weight compared to water, but at room temperatures, water holds a higher density than that dodecane and sinks in dodecane.