Nuclear notation refresher: if an element has mass number A and atomic number Z, what is the number of neutrons present in one atom of that element?

Introduction / Context:
Mass number (A) and atomic number (Z) are the two integers that uniquely characterize a nuclide. A equals the total number of nucleons (protons plus neutrons) in the nucleus; Z is the number of protons. Determining the neutron count is a straightforward but critical operation in nuclear chemistry and reactor physics.

Given Data / Assumptions:

• A = total nucleons in the nucleus (protons + neutrons).
• Z = number of protons (defines the element).
• We seek the number of neutrons, N.

Concept / Approach:
By definition, A = Z + N. Rearranging gives N = A − Z. This relationship is used universally in isotope notation, reaction balancing, and neutron economy calculations in reactor design.

Step-by-Step Solution:
Write the identity: A = Z + N.Solve for N: N = A − Z.Select the choice that matches N = A − Z.

Verification / Alternative check:
Example: for 235U, A = 235, Z = 92 → N = 235 − 92 = 143, consistent with standard nuclear data tables.

Why Other Options Are Wrong:
A + Z double-counts protons and is physically meaningless as a neutron count.A alone ignores the proton contribution.Z alone counts protons, not neutrons.

Common Pitfalls:
Confusing mass number with atomic mass (a real-number mass in units); forgetting that electrons are not part of A or Z.

Final Answer:
A- Z