Basic nuclear definition: the mass number (A) of an element equals the total count of which particles in the nucleus?

Introduction / Context:
Nuclear notation uses two integers: atomic number Z and mass number A. Z identifies the element via its proton count, while A indicates the total number of nucleons in the nucleus. This distinction underpins isotope labeling, reaction balances, and neutron economy in reactor calculations.

Given Data / Assumptions:

• A represents a simple sum within the nucleus.
• Electrons orbit the nucleus and are not counted in A.
• We are discussing stable and unstable nuclides alike.

Concept / Approach:
The nucleus contains protons and neutrons. The mass number A is defined as A = Z + N, where Z is the number of protons and N is the number of neutrons. Different isotopes of the same element share Z but have different N, and therefore different A. Reaction equations conserve A and charge (Z), enabling straightforward bookkeeping of nuclear transformations.

Step-by-Step Solution:
1) Write definition: A = protons + neutrons.2) Recognize that electrons are excluded from A because they are not part of the nucleus.3) Conclude that the correct choice is “neutrons plus protons (nucleons).”

Verification / Alternative check:
Isotope symbols like 235U (Z = 92, N = 143) demonstrate that A (235) equals 92 + 143, reinforcing the definition.

Why Other Options Are Wrong:
Electrons: not nuclear constituents.Neutrons only or protons only: each omits part of A’s definition.

Common Pitfalls:
Confusing atomic mass (in unified mass units) with the integer mass number; assuming binding energy corrections alter A (they alter mass, not the count of nucleons).

Final Answer:
Neutrons plus protons (nucleons)