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Given the atomic masses S(32.1), Se(79.0) and Te(127.6), do these three elements form a Dobereiner triad?
ANo, because the mean of S and Te is far below Se
BYes, because the mean of S and Te is close to Se
CNo, because Se is the lightest of the three
DYes, because all three are metals
Answer & Solution
Correct answer: B. Yes, because the mean of S and Te is close to Se
1. A triad requires the middle mass to equal the mean of the outer two.
2. Mean $= \frac{32.1 + 127.6}{2} = \frac{159.7}{2} = 79.85$.
3. This is very close to the actual mass of Se (79.0), so it qualifies.
4. Rule out A: 79.85 is close to 79.0, not far below it.
5. Se is the middle (not lightest) element, so C is wrong.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 2 "Periodic Classification of Elements", p.25_
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