Isotopes share protons but differ in neutrons

Steps:

• Isotopes are variants of the same element, defined by the same atomic number.
• Atomic number equals the number of protons, so isotopes have identical protons.
• Different isotopes have different mass numbers due to varying neutrons.
• Nucleons (protons + neutrons) thus differ, and electrons orbit outside the nucleus.

Why A is correct:

• Isotopes of the same element must have the same atomic number, meaning the same number of protons in the nucleus.

Why the others are wrong:

• B: Isotopes differ precisely in the number of neutrons, causing different mass numbers.
• C: Electrons are not in the nucleus; they surround it, and the statement misplaces them.
• D: Nucleons total protons plus neutrons, which vary between isotopes.

Final answer: A