MCQ practice · SSC Chemistry Board questions · Chapter 3: Structure of Matter
How many neutrons does 'A' have?
Think it through first, then pick an option.
- a 18
- b 19
- c 20
- d 39
Answer and explanation
Correct answer: c 20
Neutrons = A − Z = 39 − 19 = 20; the charge does not change the neutron count.
Why the other options are wrong
- a 18: 18 is the ion's electron count (19 − 1); neutrons = 39 − 19 = 20.
- b 19: 19 is the proton number; neutrons = 39 − 19 = 20.
- d 39: 39 is the mass number; take away the protons to get 20 neutrons.
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