What are isotopes?

Isotopes are defined as atoms of the same element that have different numbers of neutrons. This distinction in neutrons means that while isotopes of a given element have the same atomic number, which is determined by the number of protons, they differ in their mass number. The mass number is the sum of the protons and neutrons in the nucleus of an atom. For example, carbon has several isotopes, including carbon-12 (6 protons and 6 neutrons) and carbon-14 (6 protons and 8 neutrons). This difference in neutrons can lead to variations in physical properties and stability, as seen with radioactive isotopes.

The other options correctly highlight misunderstandings about what isotopes are, such as having different numbers of protons, which would indicate different elements altogether, or asserting that isotopes are identical in mass and charge.