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Question

Which of the following can form a Döbereiner's triad with lithium (atomic mass = 6.9 u) and sodium (atomic mass = 23 u)?


A

Sulphur, atomic mass = 32 u

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B

Chlorine, atomic mass = 35 u

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C

Calcium, atomic mass = 40 u

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D

Potassium, atomic mass = 39 u

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Solution

The correct option is D

Potassium, atomic mass = 39 u


  • Döbereiner arranged elements in groups of three, called triads, in the increasing order of their atomic masses. In a triad, the atomic mass of the middle element was roughly equal to the average of the atomic masses of the other two elements.
  • If lithium and sodium form a Döbereiner's triad with an element X, Atomic mass of sodium=Atomic mass of Lithium+Atomic mass of element X2
23 =6.9+Atomic mass of X2
Atomic mass of X = 39
Hence, potassium forms a Döbereiner's triad with lithium and sodium.

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