Dobereiner’s Law of Triads: CBSE Students board students who are preparing for the exam must have an idea about Dobereiner’s Law of Triads or डॉबेराइनर के त्रिक. This is an important topic, and students must understand this by heart to score well on the exam. Since the beginning of time, there have been several attempts to organise the elements according to their qualities. Numerous theories are there to categorise the elements that emerged when the new elements were found. To support their classification, different scientists employed diverse methods and evidence. These theories were continually improved upon, which ultimately resulted in the creation of the modern periodic table. Let us examine Dobereiner’s triads, one of these antiquated theories.

Students must understand the concept of triads. It is known as triads when three distinct groups of elements with similar properties are formed. German scientist Johann Wolfgang Dobereiner initially discovered it. A specific triad of elements had comparable chemical and physical characteristics. Read on to know more about Dobereiner’s Law of Triads.

Dobereiner’s Law of Triads: Description

To follow Dobereiner’s Law of Triads, firstly, we must arrange elements in ascending order of their atomic masses. The arithmetic mean of the atomic masses of the first and third elements in a triad will equal the atomic mass of the second element in that triad. Additionally, he suggested that this law applied to other properties of components as well, and density was one such characteristic.

Various Dobereiner’s Triads 

In the year 1817, the first Dobereiner’s Triad was discovered. It was made of alkaline earth metals, including Barium, Calcium, and Strontium. Later on, three more triads came into existence. Let us look at all these triads in detail. Five Dobereiner’s triads are as follows:

• Triad 1: The alkali metals like Potassium, Lithium, and Sodium constituted this triad.

Element	Atomic Mass
Sodium	7
Lithium	23
Potassium	39

We can verify the Law of Triads for this triad. Check the details below.

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

                       = (39 + 7)/2

                       = 46/2

                       = 23

Thus, the Law of Triads is verified for this triad.

• Triad 2: This triad comprises alkaline earth metals such as Calcium, Strontium, and Barium.

Element	Atomic Mass
Strontium	87.6
Calcium	40
Barium	137

Let us verify this Law of Triads through this example too.

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

                                            = 177/2 

                                            = 87.5

The atomic mass of Strontium is very close to 87.5. Thus, the Law of Triads is verified.

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

Element	Atomic Mass
Bromine	80
Iodine	127
Chlorine	35.5

The Law of Triads can again be verified.

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

                    = (127 + 35.5)/2

                    = 162.5/2

                    = 81.25 

It is very close to Bromine’s atomic mass. Hence, the Law of Triads is verified. 

• Triad 4: This triad consisted of Tellurium, Sulfur, and Selenium.

Element	Atomic Mass
Selenium	79
Sulphur	32
Tellurium	128

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

• Triad 5: This triad was constituted of Nickel, Iron, and Cobalt.

Element	Atomic Mass
Cobalt	58.9
Iron	55.8
Nickel	58.7

The atomic mass of Cobalt is approximately 57.25 times more than the arithmetic mean of the atomic masses of Iron and Nickel.

The Dobereiner Triads: Drawbacks

Dobereiner’s triads attempted to categorise the elements according to their properties into three groups and faced various limitations. Some of which are discussed below:

1. The classification of elements in Dobereiner’s Triads became challenging and, in some cases, impossible due to the numerous new elements found in the 18th and 19th centuries.
2. New elements could not be grouped in Dobereiner’s Triads, and only 5 Dobereiner’s Triads could be detected.
3. When Dobereiner’s triads were formed, several other parts did not fit into his triads. It led to a disorganised collection of elements.

Sample Questions

Check out the sample questions on Dobereiner’s Law of Triads available below.

Question 1: What exactly is known as the Law of Triads?

Answer: According to the Law of Triads, the arithmetic mean of the atomic masses of the first and third elements in a triad is roughly equal to the atomic mass of the second element in that triad when the elements are ordered in ascending order of their atomic masses.

Question 2: In relation to Dobereiner Triads, what do you mean by the word- Triad?

Answer: Triad refers to a collection of three elements when referring to Dobereiner’s Triads. Dobereiner claimed that the elements might be grouped into three with similar physical and chemical characteristics.

Question 3: Give any two of Dobereiner’s Triad constraints.

Answer: The following were Dobereiner’s Triads’ limitations:

• The 18th and 19th centuries saw the discovery of a large number of new elements, which made it challenging and occasionally impossible to classify the components in Dobereiner’s Triads.
• New elements could not be grouped in Dobereiner’s Triads, and only 5 Dobereiner’s Triads could be detected.

Question 4: Which halogens were a member of Dobereiner’s Triads?

Answer: Chlorine, Bromine, and Iodine together constituted the Dobereiner’s Triad, a group of halogens.

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

Answer: The arithmetic mean of the atomic masses of the first and third elements must be about equal to the atomic mass of the second element to form a Dobereiner trio. The atomic mass of Nitrogen is equal to the arithmetic mean of the atomic masses of Carbon and Oxygen in this situation, which is 14. Hence, they make up Dobereiner’s trio.

Question 6: Give the alkaline earth metal Dobereiner’s Triad.

Answer: Calcium, Strontium, and Barium are the alkaline earth metals that make up the Dobereiner Triads.

Question 7: Verify the alkali metals triad’s compliance with the Law of Triads.

Answer: Lithium is one of the alkali elements in the triad, along with Potassium and Sodium. These elements have atomic masses of 7, 39, and 23, respectively. The mathematical mean of the masses of Potassium and Lithium is equal to 23, which is the atomic mass of Sodium. Therefore, the Law of Triads is verified.

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