What is the iron law of the Triad?
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. Show
Dobereiner’s triads
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 TriadsThe 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:
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.
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.
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.
The arithmetic mean of atomic masses of Sulphur and Tellurium is 80 which is very close to the atomic mass of Selenium.
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:
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 QuestionsQuestion 1: What is the law of triads? Answer:
Question 2: What do you mean by triad in reference to Dobereiner triads? Answer:
Question 3: State any two limitations of Dobereiner’s triads. Answer:
Question 4: Which halogens were a part of Dobereiner’s triads? Answer:
Question 5: Verify if Carbon, Nitrogen and Oxygen with an atomic mass of 12, 14 and 16 form Dobereiner’s triads. Answer:
Question 6: State the alkaline earth metal Dobereiner’s triad. Answer:
Question 7: Verify the law of triads for the alkali metals triad. Answer:
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