Which of the following accurately defines an isotope?

An isotope is defined as atoms that have the same number of protons but differ in the number of neutrons. This definition is crucial because the number of protons determines the element's identity, while the variation in neutrons leads to different isotopes of that element, which can have different mass numbers. For instance, carbon has several isotopes, including carbon-12 (with 6 protons and 6 neutrons) and carbon-14 (with 6 protons and 8 neutrons). Both isotopes are still carbon due to their identical proton count, but they differ in their neutron count, influencing their nuclear properties and stability.

The other options do not accurately capture the definition of isotopes. Different numbers of protons would represent different elements entirely. Atoms with the same mass number may not be isotopes as they could have different arrangements of protons and neutrons. Additionally, while isotopes of an element generally have similar chemical behavior due to the same number of electrons, there can be instances where their chemical properties differ slightly because of the differences in neutron count affecting molecular interactions.