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Which pair of elements created an ambiguity in their sequence because they share the same whole-number atomic mass?
ACalcium and strontium
BSodium and potassium
CChlorine and bromine
DCobalt and nickel
Answer & Solution
Correct answer: D. Cobalt and nickel
1. Mendeleev ordered elements by increasing atomic mass.
2. Cobalt (Co) and nickel (Ni) have the same whole-number atomic mass.
3. So their order in the table could not be decided unambiguously.
4. Rule out the other pairs, whose atomic masses differ clearly.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 2 "Periodic Classification of Elements", p.29_
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