Which of the following triads was not given by Dobereiner?

Correct — C, N, P, As. Dobereiner's rule was arithmetic: in a triad of three chemically similar elements arranged by increasing atomic mass, the mass of the middle element is close to the average of the other two. Test the option — nitrogen is about 14, phosphorus about 31 and arsenic about 75, and the average of 14 and 75 is 44.5, nowhere near phosphorus's 31. The grouping therefore fails the very test that defines a triad. The other three all pass: calcium 40, strontium 88, barium 137 gives an average of 88.5 against strontium's 88; chlorine 35.5, bromine 80, iodine 127 gives 81.25 against bromine's 80; and beryllium 9, magnesium 24, calcium 40 gives 24.5 against magnesium's 24.

• (a)Ca, Sr, Ba — One of the three triads named in the school textbook, and historically the first relationship Dobereiner noticed, in 1817, when he found strontium's properties and mass sitting between calcium's and barium's.
• (b)Cl, Br, I — The halogen triad, also in the textbook list. The average of chlorine's 35.5 and iodine's 127 is 81.25, against bromine's actual 80.
• (d)Be, Mg, Ca — Satisfies the arithmetic cleanly — the average of beryllium's 9 and calcium's 40 is 24.5 against magnesium's 24 — so it cannot be the grouping singled out as failing. All four elements here are alkaline earth metals with genuinely similar chemistry.

Johann Wolfgang Dobereiner, working in the 1810s and 1820s, made the first systematic attempt to find order among the elements. He grouped chemically similar elements in threes and observed that the middle member's atomic mass approximated the arithmetic mean of the outer two. The scheme covered only a handful of elements and collapsed when applied more widely, but it established the crucial idea that atomic mass and chemical property are related — the idea Newlands's law of octaves and then Mendeleev's periodic table would build on.

Two options here can be settled from memory because they appear in every textbook list of Dobereiner's triads, and that leaves a choice between two that do not. The arithmetic then decides it, and it is worth doing rather than guessing: beryllium, magnesium and calcium pass the mean test and are chemically alike, while nitrogen, phosphorus and arsenic fail it badly. Note also that nitrogen, phosphorus and arsenic do sit in one group of the modern table, so chemical similarity alone is not enough — Dobereiner's condition was numerical.

• A Dobereiner triad requires the middle element's atomic mass to be near the mean of the other two.
• Lithium-sodium-potassium, calcium-strontium-barium and chlorine-bromine-iodine are the triads school textbooks list.
• Dobereiner identified the calcium-strontium-barium relationship in 1817 and published his triads in 1829.
• The scheme failed because only a few of the elements then known could be fitted into triads.
• Newlands's law of octaves came next, and Mendeleev's periodic law followed in 1869.

• Answering from chemical similarity alone; nitrogen, phosphorus and arsenic are group-mates but fail the mass test.
• Assuming only the three textbook triads ever existed and marking any unfamiliar grouping as false.
• Forgetting that the test uses atomic mass, not atomic number.

A history-of-chemistry item that becomes a one-line calculation once Dobereiner's actual rule is recalled.

No directly related past PYQ was found.

• practice — not a real PYQ

  In a Dobereiner triad, the atomic mass of the middle element is approximately

  • (a)the sum of the other two
  • (b)the arithmetic mean of the other two
  • (c)the difference of the other two
  • (d)half the lighter of the other two

  Answer(b) the arithmetic mean of the other two — this is the numerical rule that defines a triad.

• practice — not a real PYQ

  Which one of the following classifications immediately preceded Mendeleev's periodic table?

  • (a)Dobereiner's triads
  • (b)Newlands's law of octaves
  • (c)Moseley's atomic number
  • (d)Bohr's model of the atom

  Answer(b) Newlands's law of octaves — proposed in 1866, it followed the triads and preceded Mendeleev's 1869 periodic law.