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

Rajshahi Board 2021

An atom of an element has a protons, b electrons and c neutrons. What is its mass number?

Think it through first, then pick an option.

  1. a a + b
  2. b a + c
  3. c b + c
  4. d a + b + c
Answer and explanation

Correct answer: b a + c

Mass number = protons + neutrons = a + c.

Why the other options are wrong

  • a a + b: Electrons are not counted in the mass number; mass number = protons + neutrons = a + c.
  • c b + c: b is the electron count; the mass number adds protons (a) and neutrons (c).
  • d a + b + c: Only particles in the nucleus count; the electrons (b) are left out, giving a + c.