MCQ practice · SSC Chemistry Board questions · Chapter 3: Structure of Matter
How many electrons are in the M shell in the configuration of Co²⁺?
Think it through first, then pick an option.
- a 15
- b 14
- c 13
- d 10
Answer and explanation
Correct answer: a 15
Co²⁺ (25 electrons): ...3s² 3p⁶ 3d⁷, so M = 2 + 6 + 7 = 15.
Why the other options are wrong
- b 14: 14 would mean a 3d electron left; Co²⁺ loses only its 4s².
- c 13: 13 would mean two 3d electrons left; electrons leave 4s first.
- d 10: 10 is the full 3d capacity; Co²⁺ has only 3d⁷, so M holds 15.
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