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In Dobereiner's triads, the atomic mass of the middle element is approximately equal to which value?
AThe mean of the atomic masses of the other two elements
BThe sum of the atomic masses of the other two elements
CThe product of the atomic masses of the other two elements
DThe difference of the atomic masses of the other two elements
Answer & Solution
Correct answer: A. The mean of the atomic masses of the other two elements
1. Dobereiner grouped elements with similar properties in threes.
2. He arranged each triad in increasing order of atomic mass.
3. The middle element's atomic mass is about the mean (average) of the first and third.
4. Example: for Li(6.9), Na(23.0), K(39.1), mean $= \frac{6.9 + 39.1}{2} = 23.0$, matching Na.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 2 "Periodic Classification of Elements", p.25_
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