Isotopes share atomic number but differ in neutron count

Steps:

• Isotopes of an element have the same atomic number (Z), meaning identical protons in the nucleus.
• They differ in mass number (A), due to varying neutrons, so total nucleons (protons + neutrons) differ.
• In uncharged atoms, electrons equal protons to maintain neutrality.
• Thus, neutral atoms of isotopes have identical electron counts, as Z is the same.

Why D is correct:

• By definition, isotopes have the same atomic number Z, so uncharged atoms have Z electrons each.

Why the others are wrong:

• A: Isotopes have different neutron numbers, as that's what distinguishes them.
• B: Nuclei have different total nucleons, since mass numbers (A) vary.
• C: Nuclei contain only protons and neutrons, never electrons.

Final answer: D