Which pair represents a set of isotopes?

Question

BlackTom AI · Accepted Answer

To determine isotopes, we need two nuclei that belong to the same element (same atomic number Z) but have different mass numbers A.
Option 1: Na+ and K+ are ions of different elements (sodium vs potassium) with different atomic numbers, so they are not isotopes.
Option 2: 74Se and 82Se are both selenium atoms (Z = 34) but with different total numbers of nucleons (A = 74 and A = 82). This fits the definition of isotopes.
Option 3: Chlorine with 18 electrons (Cl–) and potassium with 18 electrons (K+) are isoelectronic species; they have the same electron count but different elements and different atomic numbers, so they are not isotopes.
Option 4: Calcium-48 and titanium-48 have the same mass number (A = 48) but belong to different elements (different Z), so they are not isotopes. In summary, the key test is same element (same Z) with different mass numbers (A), which only Option 2 satisfies.

Which pair represents a set of isotopes?单项选择题

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