Question

The atomic masses of three elements A, B, and C have similar chemical properties are 7, 23, and 39 respectively.
(a) Calculate the average atomic mass of elements A and C.
(b) Compare the average atomic mass with an atomic mass of B.
(c) What could be the elements A, B, and C?

Answer 1

Hint: A German scientist named Dobereiner states that, when elements are arranged into groups of three in the order of their increasing atomic mass, the atomic mass of the element which comes in the middle is the arithmetic mean of the rest of two. On this basis, the three elements in one group are known as ‘Triad’. This arrangement of elements is known as Dobereiner Triads.

Complete answer:
Given, the atomic mass of A, B, and C are 7, 23, and 39.
(a) Calculate the average atomic masses of elements A and C?
Atomic mass of A = 7 and C = 39
The average atomic mass of elements A and C = $\frac{A+C}{2}=\frac{7+39}{2}=\frac{46}{2}=23$
Hence, the average atomic mass of elements A and C is “B”.
(b) Compare the average atomic mass with an atomic mass of B.
The average atomic mass of elements A and C is the atomic mass of element “B”.
(c) What would be the element A, B, and C?
The atomic mass of element A is 7, which the atomic mass of lithium, hence, element A = Lithium (Li).
The atomic mass of element B is 23, which the atomic mass of sodium, hence, element B = sodium (Na).
The atomic mass of element C is 39, which the atomic mass of potassium, hence, element C = potassium (K).
In these three elements Li, Na and K are Dobereiner Triads.

Note:
Other than Li, Na, and K, elements groups like Ca, Sr, and Ba; Cl, Br, and iodine are other examples for Dobereiner Triads. Only a group of three elements in the periodic table comes under Dobereiner Triads. This rule cannot apply for elements such as very low or high atomic mass such as F, Cl, and Br group is not a Dobereiner Triads, due to the increasing order of their atomic masses and mainly atomic mass of Cl is not an arithmetic mean of F and Br.