Key properties of atoms, ions, and isotopes

Steps:

• Define isotopes as atoms of the same element with identical protons but different neutrons, confirming A.
• Note that ions form by gaining/losing electrons, not changing neutrons, so B is false.
• Recall ion charge depends only on proton-electron difference, ignoring neutrons, so C is incorrect.
• Recognize protons determine the element while neutrons vary, so D is false.

Why A is correct:

• Isotopes are defined by having the same atomic number (protons) but different mass numbers due to varying neutrons.

Why the others are wrong:

• B: Atoms and ions of the same element retain the same number of neutrons.
• C: Ion charge equals protons minus electrons; neutrons are neutral and irrelevant.
• D: Protons and neutrons differ in most atoms, as seen in isotopes.

Final answer: A