Question

Lithium, sodium and ____ are Dobereiner's triads.

• A. S
• B. Ca
• C. Cl
• D. K

Answer 1

The Correct Option is D

Dobereiner's triads were groups of three elements with similar chemical properties where the atomic mass of the middle element was approximately the average of the atomic masses of the first and third elements. Lithium (Li) and Sodium (Na) are in the same group (Group 1, the alkali metals). 

That element should also be in Group 1. Potassium (K) is the next element in Group 1 after Sodium. 

The atomic masses are approximately: 

Li: 7  Na: 23 
K: 39 Average of Li and K: $\frac{7 + 39}{2} = \frac{46}{2} = 23$ 

The average atomic mass of Li and K is approximately equal to the atomic mass of Na. 

Therefore, Li, Na, and K form a Dobereiner's triad. 

Final Answer: The final answer is $\boxed{K}$