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Q102 (CAPF/2025) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Periodic table elements Answer Verified

Which of the following triads was **not** given by Dobereiner?

Result
Your answer: —  ·  Correct: C
Explanation

Johann Wolfgang Döbereiner proposed the Law of Triads, stating that the atomic mass of the middle element in a triad is approximately the arithmetic mean of the other two.

  • Ca, Sr, Ba: Mean of Ca (40.1) and Ba (137.3) is approx. 88.7, close to Sr (87.6). (Valid)
  • Cl, Br, I: Mean of Cl (35.5) and I (126.9) is approx. 81.2, close to Br (79.9). (Valid)
  • Be, Mg, Ca: Mean of Be (9.0) and Ca (40.1) is 24.55, very close to Mg (24.3). (Mathematically consistent)
  • N, P, As: Mean of N (14.0) and As (74.9) is 44.5. This is significantly different from the atomic mass of Phosphorus (31.0).

Because the N, P, As group fails the arithmetic mean rule, it is not considered a Dobereiner triad.

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