What are isotopes?

Isotopes are defined as atoms of the same element that have different numbers of neutrons. This means that while they retain the same atomic number—and thus the same chemical properties—these variants differ in mass due to the variation in neutron count. For instance, carbon has isotopes like carbon-12, which has 6 neutrons, and carbon-14, which has 8 neutrons. Despite being the same element, their differing neutron numbers lead to differences in mass and stability. This property is significant in fields like radiometric dating, where isotopes are used to determine the age of materials.

The other choices do not accurately define isotopes. Atoms of different elements with equal atomic numbers describe a concept unrelated to isotopes. Atoms sharing electrons describes covalent bonding, which is a chemical interaction rather than a definition of isotopes. Lastly, the characteristic of having equal numbers of protons and electrons is true for neutral atoms in general, not specifically for isotopes.