MCQ Detailed View

One of the earliest attempts to classify the chemical elements was made by the German chemist Johann Wolfgang Döbereiner in 1829. He observed that certain groups of three elements, called triads, showed similar chemical properties and that the atomic weight of the middle element was approximately the arithmetic mean of the atomic weights of the other two. This observation is known as Döbereiner’s Law of Triads.

An important example of this law is seen with the halogens chlorine (Cl), bromine (Br), and iodine (I). These three elements form a classic triad. The atomic weight of chlorine is about 35.5, iodine is about 127, and bromine lies almost midway at around 80. Similarly, their chemical properties show gradual variation. Chlorine is a greenish-yellow gas, bromine is a reddish-brown liquid, and iodine is a violet-black solid at room temperature. Yet, they all form similar types of compounds, especially halide salts.

Dobereiner’s Law of Triads was also observed in other groups like alkali metals (lithium, sodium, potassium) and alkaline earth metals (calcium, strontium, barium). However, it could not be applied universally to all known elements, as not every element could be grouped into triads.

Despite its limitations, the Law of Triads was an important step in the development of the periodic classification of elements. It introduced the idea that chemical properties are related to atomic weights and that elements show periodic similarities. This concept laid the groundwork for later contributions, including Newlands’ Law of Octaves, Meyer’s atomic volume curves, and finally, Mendeleev’s Periodic Table.

Thus, the Law of Triads is applicable to the set of elements chlorine, bromine, and iodine (Cl, Br, I), which perfectly illustrate this periodic relationship.