What are isotopes?

Isotopes are defined as atoms of the same element that have the same number of protons but differ in the number of neutrons, which results in different mass numbers. This means that while isotopes share chemical properties due to having the same atomic structure (number of protons), they can display different physical properties, such as stability and nuclear behavior.

For example, carbon has several isotopes, including carbon-12 and carbon-14. Both isotopes have 6 protons, but carbon-12 has 6 neutrons, while carbon-14 has 8 neutrons. This difference in neutron count leads to varying mass numbers and can affect the isotopes' behavior in nuclear reactions.

The other options describe different concepts. Atoms with different atomic numbers refer to different elements entirely, which does not align with the definition of isotopes. Atoms that have gained or lost electrons are ions, not isotopes, as ions are characterized by their charge rather than their mass. Atoms with identical mass numbers would indicate that they are the same isotope, which also contradicts the definition of isotopes having different mass numbers.