An isotope has atomic number Z = 11 and mass number A = 23. How many neutrons does it have?

• A. 11
• B. 12
• C. 23
• D. 22

Answer: (B)

Neutrons are found by subtracting the atomic number from the mass number, because the mass number counts protons and neutrons together, while the atomic number counts only protons. With A = 23 and Z = 11, the neutrons equal 23 − 11 = 12. So this isotope has 12 neutrons. It corresponds to sodium-23 (11 protons, 12 neutrons, total 23 nucleons). If you tried 11 neutrons, the mass number would be 22; 23 neutrons would give a mass number of 34; 22 neutrons would give 33. These don’t match A = 23, so 12 is the correct count.