Mass number minus atomic number equals which quantity?

Mass number minus atomic number equals which quantity?

• A. Number of neutrons
• B. Mass number
• C. Atomic number
• D. Isotopes

Answer: (A)

Mass number counts protons plus neutrons, while the atomic number counts only protons. Subtracting the atomic number from the mass number leaves just the neutrons. So the difference tells you how many neutrons are in the nucleus. For example, if A = 23 and Z = 11, there are 12 neutrons. This also shows why isotopes exist: they have the same number of protons but different numbers of neutrons, leading to different mass numbers.

Mass number counts protons plus neutrons, while the atomic number counts only protons. Subtracting the atomic number from the mass number leaves just the neutrons. So the difference tells you how many neutrons are in the nucleus. For example, if A = 23 and Z = 11, there are 12 neutrons. This also shows why isotopes exist: they have the same number of protons but different numbers of neutrons, leading to different mass numbers.