Which of the following best describes an Isotope?

Question

BlackTom AI · Accepted Answer

Isotopes are atoms that share the same atomic number (number of protons) but differ in their neutron count, so they are different forms of the same element with different mass numbers.

1) Relevant concepts:
- Atomic number (Z) = number of protons; this defines the chemical element.
- Neutron number (N) = number of neutrons; together with protons gives mass number A = Z + N.
- Isotopes: same Z, different N. Related terms: isotones (same N, different Z) and isobars (same A, different Z and N).

2) Step-by-step reasoning:
- If two atoms have the same number of protons, they are the same element (e.g., both are carbon because they each have Z = 6).
- If their neutron counts differ, their mass numbers A differ, producing distinct isotopes of that element.
- Therefore the defining criterion for isotopes is identical proton count with varying neutron count.

3) Example calculation / intermediate steps:
- Carbon-12: Z = 6 (protons), N = 6 (neutrons) → A = 6 + 6 = 12.
- Carbon-14: Z = 6 (protons), N = 8 (neutrons) → A = 6 + 8 = 14.
- Both are carbon because Z = 6, but they are different isotopes because N differs (6 vs 8).

4) Why the correct answer is correct:
- “An atom with the same number of protons but different neutrons” exactly states that isotopes share Z but have different N, matching the standard definition in chemistry and nuclear physics.

5) Why the other options are incorrect:
- “An atom with the same number of neutrons and protons” describes atoms where N = Z (a special case like 12C), not the concept of isotopes.
- “An atom with different number of protons and same neutrons” describes isotones (same N, different Z), which are different elements, not isotopes.
- “An atom with different number of protons and neutrons” is too general (could be a different element or entirely different nuclide) and does not capture the defining condition that protons are the same.

6) Conclusion:
The correct choice is “An atom with the same number of protons but different neutrons,” which matches the provided answer.

Which of the following best describes an Isotope?单项选择题

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