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Why did the discovery of isotopes pose a problem for Mendeleev's periodic table?
AIsotopes have different properties but the same atomic mass
BIsotopes have the same properties but different atomic masses
CIsotopes have no atomic mass at all
DIsotopes belong to different groups despite similar masses
Answer & Solution
Correct answer: B. Isotopes have the same properties but different atomic masses
1. Mendeleev's table arranged elements strictly by atomic mass.
2. Isotopes of an element share chemical properties but differ in atomic mass.
3. By the mass rule they should occupy different places, yet they are one element.
4. This contradiction made their placement a real challenge.
5. Rule out A: isotopes have the SAME chemical properties, not different ones.
6. Rule out D: isotopes belong to the same group, contradicting the option.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 2 "Periodic Classification of Elements", p.29_
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