How does the atomic weight of an isotope differ from its atomic number?

The atomic weight and atomic number serve different functions in characterizing an element and its isotopes. The atomic number is defined as the number of protons in the nucleus of an atom, which determines the element's identity and also corresponds to the number of electrons in a neutral atom. This number remains constant for all isotopes of a given element, as they all contain the same number of protons.

On the other hand, the atomic weight, which is a weighted average of the masses of all the naturally occurring isotopes of an element, can vary. Different isotopes of an element have different numbers of neutrons, which alters their atomic mass. Therefore, while the atomic number remains fixed for all isotopes, the atomic weight can differ based on the specific isotopes considered and their relative abundances.

This distinction explains why the correct choice highlights that while the atomic weight can change due to variations in the number of neutrons among isotopes, the atomic number remains constant. The correct answer effectively captures the fundamental difference between these two atomic properties.