In basic atomic structure, the mass number (A) of an atom is defined as the sum of which subatomic particles?
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Aneutrons and protons
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Bprotons and electrons
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Cneutrons and electrons
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Dboth (a) & (b)
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Eelectrons only
Answer
Correct Answer: neutrons and protons
Explanation
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:
Identify nucleus contents: protons and neutrons.Define A: A = Z (protons) + N (neutrons).Exclude electrons from mass number definition.Select the option that states “neutrons and protons.”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