Question

If A, B and C are the three elements of Dobereiner's triad, and atomic weights of A and B are 7 and 15, respectively, then the atomic weight of C is:
A. 1
B. 11
C. 23
D. 25

Answer 1

Hint- Dobereiner’s try to sort the elements into some logical order by their physical properties of an element in the periodic table. According to Dobereiner’s law the atomic mass of the middle element is equal to the arithmetic mean of the atomic masses of the other rest elements. So, we can say that when we placed elements in an ascending order of atomic masses, we obtained a group of three elements having similar properties.

Complete step-by-step answer:
According to this, the atomic mass of the middle element (B) was roughly the average of the atomic masses of the other two elements. (A and C).
German chemist Johann Wolfgang Dobereiner’s identified that triads were groups of elements with similar properties for example, we take Li, Na and K with atomic weights of these elements are 7, 23 and 39 g/mol respectively.
Now, as mentioned in question,
Atomic weights of A = 7
Atomic weights of B = 15
Atomic weights of C are needed to calculate.
Now, if we consider atomic weights of C is $M_{c.}^{}$.
Or we simply say,
Let the mass of C be $M_{c.}^{}$
$M_B^{} = \dfrac{{M_A^{} + M_C^{}}}{2}$
15=$\dfrac{{7 + M_C^{}}}{2}$
$M_C^{} = 23$
Thus, we say Atomic weight of C is 23.
Therefore, option (C) is the correct answer.

Note- The mass of an atom of an element expressed in atomic mass units or we conclude that It is nearly equivalent to the number of neutrons and protons in the atom (the mass number) or to the average number allowing for the relative abundances of various isotopes.
Remember that the average atomic mass of any element is the sum of the masses of its isotopes, each multiplied by its natural abundance.