Which of the following can form Dobereiner’s triads 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
Dobereiner 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.
From the given options, the only element that can form a Dobereiner's Triad, along with lithium and sodium is only pottassium.
If A, B and C are Dobereiner's Triads, Atomic mass of B=Atomic mass of A+Atomic mass of C2