Question

Identify the set of elements that will form Dobereiner triad.

• A. Lithium, sodium, potassium
• B. Nitrogen, phosphorus, arsenic
• C. Helium, neon, argon
• D. Helium, hydrogen, potassium

Answer 1

The correct option is A Lithium, sodium, potassium

According to Dobereiner, when the three elements in a triad are written in the increasing order of atomic masses, the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements. When we apply this to the given options, then we have:

(A) Atomic mass of lithium = 7, sodium = 23 and potassium = 39

$⟹\frac{(7+39)}{2}=23$

(B) Atomic mass of helium = 4, neon = 20 and argon = 40

$⟹\frac{(4+40)}{2}=22$

(C) Atomic mass of nitrogen = 14, phosphorus = 31 and arsenic = 75

$⟹\frac{(14+75)}{2}=44.5$

(D) Atomic mass of lithium = 7, calcium = 40 and potassium = 39

$⟹\frac{(7+39)}{2}=23$
In the given options, only the set consisting of lithium, sodium, and potassium is satisfying Dobereiner's triad.