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Dobereiners Triads – Definition, Types, Limitations

  • Last Updated : 21 Sep, 2021

Since ancient times, there have been various attempts to classify the elements into groups according to their properties. As the new elements were discovered, a number of theories came up to classify the elements. Various scientists used various approaches and facts to justify their classification. These theories were refined more and more and finally lead to the development of the modern periodic table. Let us have a look at one of these old theories i.e. Dobereiner’s triads.

Dobereiner’s triads

A German chemist Johann Wolfgang Dobereiner identified that elements with similar properties could be grouped together in groups of three which were referred to as triads. The elements in a particular triad had similar chemical and physical properties. 

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Law of Triads: According to Dobereiner, when elements are arranged in increasing order of their atomic masses, the arithmetic mean of the atomic masses of the first and third element in a triad is approximately equal to the atomic mass of the second element in that triad. He also proposed that this law was valid for other properties of elements too. One such property was density. 



Various Dobereiner’s Triads 

The first Dobereiner’s triad was discovered in 1817. It is comprised of alkaline earth metals i.e. calcium, strontium and barium. Later three more triads were discovered. Let us look at these triads in detail. There are five of Dobereiner’s triads which are as follows:

  • Triad 1: This triad was constituted by the alkali metals lithium, sodium and potassium.
Element Atomic Mass

Lithium

7

Sodium

23

Potassium 

39

The law of triads can be verified for this triad as follows:



The atomic mass of Sodium = (Atomic Mass of Lithium + Atomic mass of Potassium) / 2

                                             = (7 + 39)/2

                                             = 46/2

                                             = 23

Thus the law of triads is verified for this triad.

  • Triad 2: This triad is comprised of alkaline earth metals i.e. calcium, strontium and barium.
ElementAtomic Mass

Calcium

40

Strontium 

87.6

Barium



137

Let us verify the law of triads for this triad too.

(Atomic Mass of Calcium + Atomic mass of Barium) / 2 = (40 + 137) / 2

                                                                                        = 177/2 

                                                                                        = 87.5

87.5 is very close to the atomic mass of Strontium. Thus the law of triads is verified.

  • Triad 3: This triad is comprised of halogens, Chlorine, Bromine, Iodine.

Element

Atomic Mass

Chlorine 

35.5



Bromine 

80

Iodine 

127

The Law of triads can again be verified for this triad too.

Atomic Mass of Bromine = (Atomic Mass of Chlorine + Atomic mass of Iodine) / 2

                                        = (35.5 + 127)/2

                                        = 162.5/2

                                       = 81.25 

which is very close to the atomic mass of Bromine. Hence the law of triads is verified. 

  • Triad 4: This triad consisted of sulphur, selenium, and tellurium.

Element

Atomic Mass

Sulphur

32

Selenium 

79

Tellurium 

128

The arithmetic mean of atomic masses of Sulphur and Tellurium is 80 which is very close to the atomic mass of Selenium. 

  • Triad 5: This triad was constituted of Iron, cobalt and nickel.

Element



Atomic Mass

Iron

55.8

Cobalt

58.9

Nickel

58.7

The atomic mass of Cobalt is roughly equal to the arithmetic mean of the atomic mass of Iron and Nickel that comes out to be 57.25.

Limitations of Dobereiner’s triads:

Although Dobereiner’s triads made an attempt to classify the elements according to their properties into groups of three still had various limitations which are discussed below:

  1. Many new elements were discovered in the 18th and 19th century which made the classification of elements in Dobereiner’s triads difficult and also impossible in some cases.
  2. Only 5 Dobereiner’s triads could be identified and new elements could not be grouped in Dobereiner’s triads.
  3. There also existed several elements at the time of Dobereiner’s triads formation that did not fit into Dobereiner’s triads.
  4. It resulted in disordered element collection.

Although Dobereiner’s triads could not group all elements according to their properties it said the foundation for the formation of the modern periodic table. 

Sample Questions

Question 1: What is the law of triads?

Answer:

According to the law of triads, when elements are arranged in increasing order of their atomic masses, the arithmetic mean of the atomic masses of the first and third element in a triad is approximately equal to the atomic mass of the second element in that triad.

Question 2: What do you mean by triad in reference to Dobereiner triads?

Answer:

In reference to Dobereiner’s triads, triad refers to a group of 3 elements. According to Dobereiner,  tge elements could be made into groups of three having similar physical and chemical properties. 

Question 3: State any two limitations of Dobereiner’s triads.

Answer:

Limitations of Dobereiner’s triads were as follows:

  • Many new elements were discovered in 18th and 19th century which made the classification of elements in Dobereiner’s triads difficult and also impossible in some cases.
  • Only 5 Dobereiner’s triads could be identified and new elements could not be grouped in Dobereiner’s triads.

Question 4: Which halogens were a part of Dobereiner’s triads?



Answer:

Dobereiner triad that consisted of halogens was formed by Chlorine, Bromine and Iodine. 

Question 5: Verify if Carbon, Nitrogen and Oxygen with an atomic mass of 12, 14 and 16 form Dobereiner’s triads. 

Answer:

To form a Dobereiner triad, the arithmetic mean of the atomic mass of first and third element must be approximately equal to the atomic mass of the second element. 

Here arithmetic mean of the atomic mass of Carbon and Oxygen is 14 which is equal to the atomic mass of Nitrogen. Hence they form Dobereiner’s triad.

Question 6: State the alkaline earth metal Dobereiner’s triad.

Answer:

The alkaline earth metals that are a part of the Dobereiner triads are calcium, strontium and barium.

Question 7: Verify the law of triads for the alkali metals triad.

Answer:

The alkali metals triad consists of Lithium. Sodium and Potassium. The atomic masses of these elements are 7, 23 and 39 respectively. The arithmetic mean of the masses of lithium and potassium is equal to 23 which is the atomic mass of Sodium. Hence the law of triads is verified.




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