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JAMB Chemistry 1989 Question 10

Question 10

Atomic Structure

The atomic number of caesium is 55 and its atomic mass is 133. The nucleus of caesium atom therefore contains

  1. A.78 protons and 55 electrons
  2. B.55 protons and 78 neutrons
  3. C.55 protons and 78 electrons
  4. D.78 protons and 55 neutrons.
Show the answer and explanation

Answer: B (55 protons and 78 neutrons)

Explanation

Protons = atomic number = 55; neutrons = 133 − 55 = 78. Electrons are outside the nucleus.