In basic atomic structure, the mass number (A) of an atom is defined as the sum of which subatomic particles?

Introduction / Context:
The mass number is a foundational concept in chemistry and nuclear engineering. It underpins isotopic notation, nuclear reactions, and calculations involving molar mass approximations for elements and nuclides.

Given Data / Assumptions:

• Atoms contain protons, neutrons (nucleons), and electrons.
• Electron mass is negligible compared with nucleons for mass number purposes.

Concept / Approach:
Mass number A equals the total number of nucleons in the nucleus, i.e., protons (Z) plus neutrons (N). Electrons determine charge balance but contribute negligibly to A. Therefore A = Z + N, and isotopes of the same element share Z but differ in N and hence in A.

Step-by-Step Solution:

Verification / Alternative check:
Standard nuclide notation ^A_ZX encodes A and Z explicitly, confirming A counts nucleons only.

Why Other Options Are Wrong:

• Protons and electrons / neutrons and electrons: electrons do not determine mass number.
• Both (a) & (b): includes an incorrect pairing.
• Electrons only: incorrect by definition.

Common Pitfalls:
Confusing atomic number (Z = protons) with mass number (A = protons + neutrons).

Final Answer:
neutrons and protons