Elements A, B and C occur as Dobereiner's triads. If the atomic mass of A is 40 and that of C is 137, what will be the atomic mass of B?
- A.35
- B.120
- C.88
- D.74
Correct answer
Explanation
The correct answer is C, 88. Johann Wolfgang Dobereiner, a German chemist, grouped elements of similar properties in sets of three that he called triads. His rule was that the atomic mass of the middle element is about the average of the other two. Here the average of 40 and 137 is 177 divided by 2, which is 88.5, so the nearest choice is 88. The triad meant is calcium, strontium and barium, whose masses are 40, 87.6 and 137. Dobereiner found only three such triads, so his scheme covered too few elements to be useful, but it was the first hint that atomic mass and properties are linked, an idea that later led to Newlands' law of octaves and to Mendeleev's periodic table. A, B and D are wrong because 35, 120 and 74 are far from the average of the two given masses. Exam tip: in a triad the middle mass is the average of the first and third.
Practice Questions
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