Question

Chlorine,bromine and iodine form a Dobereiner's Triad .Chlorine has atomic mass 35.5 and iodine has atomic mass 126.9.Predict the atomic mass of bromine.

Answer 1

Hint: Dobereiner’s triads and Newland’s law of octaves were early attempts at classifying Dobereiner arranged the element in increasing order of atomic masses. He found that the atomic mass of the middle element was approximately equal to the arithmetic mean (average) of the atomic masses of the other two elements of that triad when they are arranged in their increasing order of atomic mass,g elements into groups based on their properties.

Complete step by step answer:
The first of Dobereiner’s triads was identified in the year 1817 and was constituted by the alkaline earth metals calcium, strontium, and barium
In Triad 1-alkali metals lithium, sodium, and potassium were there
And the arithmetic mean of the masses of potassium and lithium corresponds to 23.02, which is almost equal to the atomic mass of sodium.
In Triad 2-calcium, barium, and strontium formed another one of Dobereiner’s triads.
The mean of the masses of barium and calcium corresponds to 88.7.
In Triad 3-The halogens chlorine, bromine, and iodine constituted one of the triads
The mean value of the atomic masses of chlorine and iodine is 81.1.

The fourth triad was formed by the elements sulfur, selenium, and tellurium.
The arithmetic mean of the masses of the first and third elements in this triad corresponds to 79.85.
The Triad 5-Iron, cobalt, and nickel constituted the last of Dobereiner’s triads.
the mean of the atomic masses of iron and nickel corresponds to 57.3.

Step by step solution: Dobereiner's triads- It states that the atomic mass of middle element in doberiener's triad is exactly or nearly the arithmetic mean of the atomic masses of other two elements.
Atomic mass of Bromine = $\dfrac{(Atomic\; Mass \;Of \;Chlorine)+(Atomic\;mass\;ofIodine)}{2}$
Atomic mass of Bromine = $\dfrac{35.5+126.9}{2}$

So, Atomic mass of Bromine = 81.2.

Note: The primitive methods of classifying elements, such as Newland’s law of octaves and Dobereiner’s triads, laid the foundation for the development of the modern periodic table
Limitations of Dobereiner’s Triads
1.The key shortcomings of Dobereiner’s method of classifying elements are listed below.
2.The identification of new elements made this model obsolete.
3.Newly discovered elements did not fit into the triads.
4.Only a total of 5 Dobereiner’s triads were identified.
5.Even several known elements did not fit into any of the triads.