Question

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Answer 1

Step-by-step explanation:

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Answer 2

1. A has 2 electrons, so atomic number is 2, then element A is helium, so mass number of helium is 4
2. B has 10 electrons, so atomic number is 10
3. C and D elements represents a pair of isotope
4. C has 8 electrons, so atomic number is 8, then element C is oxygen, valency of oxygen is -2