A Level Physics Practice Exam

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In atomic structure, what does the term 'mass number' refer to?

  1. The total number of protons

  2. The total number of electrons

  3. The total number of protons and neutrons

  4. The total number of neutrons

The correct answer is: The total number of protons and neutrons

The term 'mass number' refers specifically to the total number of protons and neutrons in an atomic nucleus. This value is significant because it represents the overall mass of the atom, as protons and neutrons contribute nearly all of the atomic mass, while the mass of electrons is negligible in comparison. Understanding the mass number is key for identifying isotopes of elements, as isotopes of an element will have the same number of protons (thus the same atomic number) but different mass numbers due to varying numbers of neutrons. This aspect is crucial in fields such as chemistry and nuclear physics, where the behavior of isotopes can significantly influence reactions and stability.

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