Isotopes have the same atomic number but different mass numbers

Steps:

• Calculate atomic number Z (protons) for each nuclide: Z = total particles - nucleons, since total particles = nucleons + electrons and electrons = protons in neutral atoms.
• Compute mass number A = nucleons for each.
• Compare Z values: identical Z indicates same element; different A indicates different neutron count.
• W and Y share the same Z but have different A, confirming they are isotopes.

Why B is correct:

• Isotopes are defined as nuclides with the same atomic number Z (protons) but different mass numbers A (nucleons), per nuclear chemistry definitions.

Why the others are wrong:

• A: W and X have different Z values, so different elements.
• C: X and Z have different Z values, so different elements.
• D: Y and Z have different Z values, so different elements.

Final answer: B