Question

Which of these triads could not be justified as Dobereiner’s triad?

• A. Li, Na, K
• B. Cl, Br, I
• C. C, N, O
• D. Ca, Sr, Ba

Answer 1

The correct option is C

C, N, O

Explanation for Correct option:

C) Dobereiner's triad: When the elements are arranged in a set of 3 elements, the atomic mass of second element is the average of atomic masses of first and second elements.

C(12),N(14),O(16)- $\frac{12+16}{2}=\frac{28}{2}=14$

Though, the average of C & O is equal to the mass of N their physical and chemical properties do not resemble with each other.

Explanation for Incorrect options:

A)Li(7), Na(23), K(39) - $\frac{7+39}{2}=\frac{46}{2}=23$ These elements have similar physical and chemical properties and follow Dobereiner principle.

B)Cl(35.5),Br(80),I(!27)- $\frac{35.5+127}{2}=\frac{162.5}{2}=81.25$These elements have similar physical and chemical properties and follow Dobereiner principle.

D)Ca(40), Sr(87.6), Ba(137)-$\frac{40+137}{2}=\frac{177}{2}=88.5$These elements have similar physical and chemical properties and follow Dobereiner principle.

Hence,C) is the correct option-C,N,O could not be justified as a Dobereiner triad.