The mass of a nucleus is the

Analyze the question: It asks what determines the mass of a nucleus.

Concept: The nucleus is made up of protons and neutrons, which are together called nucleons. Electrons are not in the nucleus. They orbit around it, and their mass is about 1/1836 of a proton's mass, so it is negligible.

• Mass number (A) = number of protons (Z) + number of neutrons (N)
• The mass of a nucleus is therefore approximately the number of nucleons (protons + neutrons) multiplied by the atomic mass unit.

(Strictly, the actual mass is slightly less than the sum of the individual nucleon masses because of the mass defect, which is converted to binding energy. For JAMB-level questions, the mass of the nucleus is linked to the total number of protons and neutrons.)

Checking each option:

• A. Total number of protons and neutrons. Correct. This is the mass number, which gives the nucleus's mass.
• B. Number of its protons. This is the atomic number, which gives the charge and not the mass.
• C. Mass of its electrons. Electrons are not in the nucleus.
• D. Total number of protons and electrons. Electrons are not in the nucleus, and this leaves out the neutrons.

Answer: A.

Tip: Remember "Nucleus = Nucleons = Protons + Neutrons". Whenever you see "mass" with "nucleus", think of the mass number (A). Whenever you see "charge" or "identity of the element", think of the atomic number (Z).