Isotopes have the same number of protons.

Isotopes have the same number of protons.

• A. They have different numbers of protons
• B. They have different numbers of neutrons
• C. They have the same number of protons
• D. They have different numbers of electrons

Answer: (C)

Isotopes are variants of the same element that share the same number of protons, which fixes the element’s identity. Because the protons determine which element it is, isotopes belong to the same element even though their masses differ. The difference among isotopes comes from the number of neutrons, so they have different mass numbers while keeping the same proton count. That’s why the statement about having the same number of protons is the correct description. If you think about the other possibilities: having a different number of protons would change the element itself, not just the isotope. Having different numbers of neutrons is true for isotopes, but it’s the neutrons that vary, not the protons, which is why the proton count is the defining feature. The number of electrons would match the proton count in a neutral atom, so isotopes can share the same electron count as well; changing electrons would reflect ionization, not a change in isotope.

Isotopes are variants of the same element that share the same number of protons, which fixes the element’s identity. Because the protons determine which element it is, isotopes belong to the same element even though their masses differ. The difference among isotopes comes from the number of neutrons, so they have different mass numbers while keeping the same proton count. That’s why the statement about having the same number of protons is the correct description.

If you think about the other possibilities: having a different number of protons would change the element itself, not just the isotope. Having different numbers of neutrons is true for isotopes, but it’s the neutrons that vary, not the protons, which is why the proton count is the defining feature. The number of electrons would match the proton count in a neutral atom, so isotopes can share the same electron count as well; changing electrons would reflect ionization, not a change in isotope.