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MCQ practice · SSC Chemistry Board questions · Chapter 3: Structure of Matter

Barishal Board 2023

For the given ion ¹⁶₈O²⁻:

Think it through first, then pick an option.

  1. a Number of electrons 10
  2. b Number of neutrons 10
  3. c Number of protons 10
  4. d Mass number 14
Answer and explanation

Correct answer: a Number of electrons 10

In a negative ion, electrons = Z + charge = 8 + 2 = 10.

Why the other options are wrong

  • b Number of neutrons 10: Neutrons = 16 − 8 = 8; the charge does not change neutrons. 10 is the electron count.
  • c Number of protons 10: Protons = Z = 8; an ion keeps its protons. The 10 are electrons.
  • d Mass number 14: The mass number is the top number, 16, and does not change in an ion.