In atomic structure, what determines the mass number (A) of an element for a given atom or nuclide?

Introduction / Context:
In nuclear chemistry and atomic physics, each atom or nuclide is characterized by its atomic number and mass number. The mass number is important for distinguishing isotopes and for calculating nuclear properties. This question asks you to determine which particles inside the atom contribute to the mass number of an element and therefore define its value for a given atom.

Given Data / Assumptions:

• We consider a neutral or ionized atom in general, focusing on its nucleus.
• Atomic number (Z) is defined as the number of protons in the nucleus.
• Mass number (A) is defined in standard nuclear notation.
• Electrons have negligible mass compared with protons and neutrons.

Concept / Approach:
The mass number A of a nuclide is defined as the total number of nucleons in the nucleus. Nucleons are the particles found in the nucleus: protons and neutrons. Electrons orbit the nucleus and contribute almost nothing to the mass because their mass is much smaller than that of protons and neutrons. Therefore, the mass number is the sum of the number of protons and the number of neutrons in the nucleus. Isotopes of the same element have the same number of protons but different numbers of neutrons, and hence different mass numbers.

Step-by-Step Solution:
Step 1: Recall that protons and neutrons together are called nucleons and reside in the nucleus. Step 2: Remember that the mass number A is defined as A = number of protons + number of neutrons. Step 3: Recognise that electrons, while important for chemical properties, do not significantly contribute to the atomic mass due to their very small mass. Step 4: Compare the given options and identify the one that matches the definition of mass number as the sum of protons and neutrons. Step 5: Select “number of protons and neutrons” as the correct answer.

Verification / Alternative check:
Consider a common isotope such as carbon 12. It has 6 protons and 6 neutrons, giving a mass number A of 12. Another isotope, carbon 14, has 6 protons and 8 neutrons, giving a mass number of 14. The atomic number (number of protons) remains 6 for both, but the mass number changes because the number of neutrons changes. This demonstrates that mass number depends on both protons and neutrons, and not on electrons. Similar reasoning applies to other elements and their isotopes, confirming the correctness of the answer.

Why Other Options Are Wrong:
The number of protons alone determines the atomic number, not the mass number, so option A is incomplete. The number of electrons and protons mainly determines the charge state of the atom and its chemical behavior, but electrons are too light to significantly affect mass number, making option B incorrect. The number of neutrons alone does not identify an element, because different elements can have the same number of neutrons. Furthermore, mass number must account for both types of nucleons. Option D therefore cannot be correct for the definition of mass number.

Common Pitfalls:
Students sometimes confuse atomic number with mass number or mistakenly think that electrons contribute significantly to the mass because they are often discussed alongside protons and neutrons. Another pitfall is to forget that isotopes have different mass numbers even though they share the same number of protons. To avoid confusion, remember the simple formulas: atomic number Z = number of protons; mass number A = number of protons + number of neutrons. Keeping these definitions clear helps in solving many problems in nuclear chemistry and physics.

Final Answer:
The mass number (A) of an element is determined by the number of protons and neutrons in the nucleus of that atom.