Question

How to place the ......... in Medeleev's periodic table was a major challenge.

1 noble gas
2 isotopes
3 halogen
4 radioactive
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Answer 1

Answer:

How to place the halogen in Medeleev's periodic table was a major challenge.

Answer: 3) Halogen

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Answer 2

Answer:

How to place the .Isotopes.. in Medeleev's periodic table was a major challenge.

Explanation:

Isotopes of elements were found which violated Mendeleev’s periodic law.