CAPF AC 2025 Q102: Science & Technology – Chemistry
- (a) Ca, Sr, Ba
- (b) Cl, Br, I
- (c) N, P, As
- (d) Be, Mg, Ca
Answer: (c) N, P, As
Explanation
Dobereiner's triads
How to think: in a Dobereiner triad the atomic mass of the middle element is about the average of the other two, and the three have similar properties.
Test each option with that rule.
| Option | Masses | Average of ends | Middle | A triad? |
|---|---|---|---|---|
| a) Ca, Sr, Ba | 40, 88, 137 | 88.5 | 88 | Yes, given by Dobereiner |
| b) Cl, Br, I | 35.5, 80, 127 | 81 | 80 | Yes, given by Dobereiner |
| c) N, P, As | 14, 31, 75 | 44.5 | 31 | No, the rule fails |
| d) Be, Mg, Ca | 9, 24, 40 | 24.5 | 24 | Fits the rule |
Calcium-strontium-barium and chlorine-bromine-iodine are the textbook triads.
N, P, As does not satisfy the average rule at all, so it is the one Dobereiner did not give.
Answer: (c) N, P, As.
Why each option is right or wrong
- (a) Incorrect. Ca, Sr, Ba is one of Dobereiner's triads.
- (b) Incorrect. Cl, Br, I is one of Dobereiner's triads.
- (c) CORRECT. The mass of P is far from the average of N and As, so this is not a triad.
- (d) Incorrect. Be, Mg, Ca satisfies the rule, though it is not one of the textbook triads.
Key point
Dobereiner (1829): Li-Na-K, Ca-Sr-Ba, Cl-Br-I, S-Se-Te. Rule: middle mass is the mean of the other two. Newlands: law of octaves (1866). Mendeleev: periodic law (1869).
Remember
Triad rule: middle mass is the average. N, P, As fails it.
Common mistakes
- Choosing (d) because Be, Mg, Ca is not in the textbook list, without testing the average rule.
- Mixing Dobereiner's triads with Newlands' octaves.
- Not testing the options with the average rule.
Exam tip
Educated guess: N, P, As are far apart in mass (14, 31, 75), so the middle cannot be the average. The other options have neat averages.
Sources
- NCERT Class X Science, Chapter 5 (Periodic Classification of Elements: Dobereiner's triads)
- William H. Brock, The Fontana History of Chemistry: Dobereiner and the triads of 1829