What is an isotope?

• A. Atoms of different elements with the same mass
• B. Atoms of the same element with different masses due to different numbers of neutrons
• C. Ions of elements with different charges
• D. Molecules with the same formula but different shapes

Answer: (B)

Isotopes are atoms of the same element that have different masses because they contain different numbers of neutrons in the nucleus. The number of protons (which defines the element) is the same, so chemical behavior is largely the same, but the total mass and some nuclear properties vary. For example, carbon-12 and carbon-14 are both carbon atoms, but carbon-14 has more neutrons and is radioactive.

The other ideas mix up different concepts: atoms of different elements with the same mass would be isobars; ions with different charges are about charge states, not isotopes; and molecules with the same formula but different shapes describe isomers, not isotopes.