How many neutrons does an atom have, given its Atomic Mass Number and Proton Number?

• A. Atomic Mass Number - Proton Number
• B. Proton Number
• C. Atomic Number
• D. Atomic Mass Number + Proton Number

Answer: (A)

Neutrons can be found by subtracting the number of protons from the total number of nucleons. The mass number A counts all nucleons (protons plus neutrons), while the proton number Z tells how many protons there are. Since A = Z + N, the number of neutrons is N = A − Z. So, given the atomic mass number and the proton number, neutrons equal the mass number minus the proton number. For a quick example, if A = 23 and Z = 11, there are 12 neutrons.