Isotopes share atomic number but differ in mass number

Steps:

• Recall isotopes are atoms of the same element with identical proton counts (atomic number) but varying neutron counts (mass number).
• Examine diagrams for proton numbers: same protons indicate same element.
• Check mass numbers (protons + neutrons): differences confirm isotopes.
• Compare pairs: only 1 and 4 match on protons, differ on neutrons.

Why B is correct:

• Diagrams 1 and 4 have the same atomic number (protons) but different mass numbers, per the definition of isotopes.

Why the others are wrong:

• A: Diagrams 1 and 2 have different atomic numbers, so different elements.
• C: Diagrams 2 and 3 have different atomic numbers, so different elements.
• D: Diagrams 3 and 4 have different atomic numbers, so different elements.

Final answer: B