How to find the number of neutrons?

• A. Atomic number minus mass number
• B. The number of electrons in the atom
• C. Mass number plus atomic number
• D. Mass number minus atomic number

Answer: (D)

Neutrons are the difference between the total number of nucleons and the number of protons in the nucleus. The mass number counts protons plus neutrons, while the atomic number counts only protons. So the number of neutrons equals mass number minus atomic number (N = A − Z). For example, carbon-12 has A = 12 and Z = 6, so it has 6 neutrons. Uranium-238 has Z = 92, so N = 238 − 92 = 146 neutrons. The other options mix up what’s counted or use the wrong operation, so they don’t reflect the actual neutron count.