If an element's mass number changes but atomic number remains constant, they are isotopes.

• A. Isomers
• B. Ions
• C. Elements
• D. Isotopes

Answer: (D)

Isotopes are variants of the same element that differ in the number of neutrons. The atomic number, which is the number of protons and defines the element, stays the same, so the identity of the element doesn’t change. The mass number is Z plus the number of neutrons; if the mass number changes while the atomic number remains fixed, the neutron count has changed, giving you isotopes like carbon-12 and carbon-13. Chemically they behave similarly because chemistry depends on electrons and the proton count, but their masses and some nuclear properties differ. This idea is different from isomers (different arrangements of atoms in a molecule), ions (charged species from losing or gaining electrons), or different elements (different atomic numbers).