Isotopic masses occur because there are atoms of an element with the same number of protons but different number of neutrons.

• A. Atoms with the same number of protons but different numbers of neutrons have different masses
• B. Atoms with different numbers of protons have different masses
• C. Isotopic masses refer to charged ions
• D. Isotopic masses are the mass of molecules

Answer: (A)

Isotopes have the same number of protons but a different number of neutrons, so their total mass differs. The mass of an atom comes mainly from its protons and neutrons, while electrons contribute very little, so changing the neutron count changes the mass without changing the element’s identity. This is why isotopic masses are different for atoms of the same element. If the number of protons varied, you’d be describing a different element entirely, and isotopic masses aren’t about ions or molecules.