The atomic number of an element is 8. How many electrons will it gain to form a compound with sodium?
- A.One
- B.Two
- C.Three
- D.Four
Correct answer
Explanation
The correct answer is B, Two. The element with atomic number 8 is oxygen, which gains two electrons to complete its octet and forms sodium oxide, Na₂O.
Oxygen's electrons are arranged as 2, 6, so it needs two more to reach the stable arrangement 2, 8 of neon. Sodium (2, 8, 1) readily loses its single outer electron. Two sodium atoms therefore give one electron each to an oxygen atom, forming two Na⁺ ions and one O²⁻ ion held together by ionic bonds. This is why the valency of oxygen is 2.
A is wrong: gaining one electron would leave oxygen with seven outer electrons, short of an octet; elements such as chlorine gain one.
C is wrong: gaining three electrons, as nitrogen does, would give oxygen nine outer electrons.
D is wrong: four is far too many; carbon, not oxygen, completes its octet by sharing four electrons.
Exam tip: O = 2, 6, valency 2, forms O²⁻; Na = 2, 8, 1, forms Na⁺; together they make Na₂O.
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