Question

According to the law of triads:
A. The properties of the middle element were in between those of the other two members.
B. Three elements arranged according to increasing weights have similar properties.
C. The elements can be grouped in the groups of six elements.
D. Every third element resembles the first element in the periodic table.

Answer 1

We need to first understand the law of triads. With the discovery of many new elements in the 18 and 19 century, several laws and theories were brought up to sort these elements in the periodic table. Earlier elements in the periodic table were classified into two groups as metals and non-metals only. But later on, experiments were conducted to classify the elements in a more detailed manner. Dobereiner’s triads was one law that laid the foundation of the modern periodic table.

Complete step by step answer:
In the history of the classification of elements in the periodic table, a German Chemist named Dobereiner’s triads were an early attempt to sort the elements in an order based on their similar physical and chemical properties. A triad is basically a consecutive series of three elements. Dobereiner’s law of triads stated that the arithmetic mean of the atomic masses of the first and third element in a triad would be approximately equal to the atomic of the second element in that triad. In other words, the atomic mass of the middle element of a triad is the approximate average of the preceding and the succeeding element. Hence, the properties of the three elements of a triad were found to be similar and the properties of the middle element were intermediate of the other two members.

So, the correct answer is Option A.

Note: It must be noted that Dobereiner identified five triads in the periodic table. The first triad which was identified was composed of three alkaline earth metals or halogens calcium, strontium, and barium. The atomic mass of calcium is $40.1$ and that of barium is$137.3$ the mean of which is $88.7$ which approximately corresponds to the atomic mass of strontium which is $87.6$. One major limitation of the Dobereiner’s triads was that many known elements did not fit into any of the triads.