Which of the following triads is NOT a Dobereiner's triad?
(i) Li, Na, K
(ii) Ca, Sr, Ba
(iii) N, P, Sb
(iv) Cl, Br, I

Correct option is C

The correct Answer is (c) Only iii

​Dobereiner's triad:

A triad consists of three elements with similar chemical properties.
The atomic mass of the middle element is approximately the average of the atomic masses of the other two elements.

Analysis of the Given Triads:

i) Li, Na, K:
Atomic masses: Li = 7, Na = 23, K = 39.
Average of Li and K: (7+39)/2 = 23which is equal to Na's atomic mass.
This is a Dobereiner's triad

​(ii) Ca, Sr, Ba:

​Atomic masses: Ca = 40, Sr = 88, Ba = 137.

Average of Ca and Ba: (40+137)/2=88.5(40 + 137)/2 = 88.5(40+137)/2 = 88.5 approximately equal to Sr's atomic mass.

This is a Dobereiner's triad

​(iii) N, P, Sb:

​Atomic masses: N = 14, P = 31, Sb = 122.

Average of N and Sb: (14+122)/2=68(14 + 122)/2 = 68(14 + 122)/2 = 68 which is not close to P's atomic mass.

This is NOT Dobereiner's triad

​(iv) Cl, Br, I:

​Atomic masses: Cl = 35.5, Br = 80, I = 127.

Average of Cl and I:(35.5+ 127)/2 = 81.25  approximately equal to Br's atomic mass.

This is a Dobereiner's triad