Which statement describes isotopes?

• A. An element's atom with a different number of protons
• B. An ion of an element
• C. A molecule
• D. An isotope is an element's atom with a different number of neutrons

Answer: (D)

Isotopes are atoms of the same element that differ in the number of neutrons they contain. They keep the same number of protons, so the element’s identity (its atomic number) is unchanged, but the mass changes because neutrons add mass without changing charge. For example, carbon-12 and carbon-14 have six protons, but different numbers of neutrons, giving them different mass numbers. Because the protons (and electrons) largely determine chemical behavior, isotopes of the same element behave similarly chemically, though their nuclei differ in stability and some physical properties.

If you change the number of protons, you’ve created a different element entirely, not an isotope. If you change the number or arrangement of electrons, you get an ion, which is charged but not defined by a different neutron count. And a molecule is simply a bonded collection of atoms, not a description of a single atom’s neutron content.