Which quantity tells us the number of protons in the nucleus?

Which quantity tells us the number of protons in the nucleus?

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

Answer: (B)

The number of protons in the nucleus is given by the atomic number. This quantity, often denoted Z, is what defines the element itself, since each element has a unique number of protons. The mass number A is the total of protons and neutrons (A = Z + N), so it includes neutrons too. The number of neutrons is N = A − Z, and the electron count only directly matches the proton count in neutral atoms (ions can differ). So the atomic number is the quantity that tells us exactly how many protons are present.

The number of protons in the nucleus is given by the atomic number. This quantity, often denoted Z, is what defines the element itself, since each element has a unique number of protons. The mass number A is the total of protons and neutrons (A = Z + N), so it includes neutrons too. The number of neutrons is N = A − Z, and the electron count only directly matches the proton count in neutral atoms (ions can differ). So the atomic number is the quantity that tells us exactly how many protons are present.