Define isotopes in terms of atomic structure.

• A. Atoms with the same number of neutrons but different protons
• B. Atoms of the same element with different numbers of neutrons
• C. Atoms with the same mass but different atomic numbers
• D. Atoms that have lost or gained electrons

Answer: (B)

Isotopes are specifically defined as atoms of the same element that have the same number of protons but different numbers of neutrons. This means that while isotopes share the same atomic number (which defines the element itself), they will have different mass numbers due to the variation in neutrons.

For example, carbon has isotopes such as carbon-12 and carbon-14. Both isotopes have 6 protons (which is what makes them carbon), but carbon-12 has 6 neutrons, while carbon-14 has 8 neutrons. This difference in the number of neutrons leads to different physical properties, such as varying stability and different behaviors in nuclear reactions.

The other options do not accurately define isotopes. Atoms with the same number of neutrons but different numbers of protons would represent different elements entirely. Atoms with the same mass but different atomic numbers would also not be the same element, as the atomic number is what identifies the element. Finally, the option describing atoms that have lost or gained electrons refers to ions, not isotopes, since ions are defined by their charge due to the change in electron number rather than the change in neutrons.