What is an isotope?

An isotope refers to atoms that share the same number of protons, which means they belong to the same element and have the same atomic number, but differ in the number of neutrons present in their nuclei. This variation in neutron count leads to different mass numbers for these isotopes of an element. For example, carbon-12 and carbon-14 are two isotopes of carbon. Both have six protons, but carbon-12 has six neutrons while carbon-14 has eight neutrons. This distinction is essential in various fields such as chemistry, physics, and nuclear medicine, as isotopes can exhibit different nuclear stability, radioactive properties, and applications, yet maintain similar chemical behavior due to their identical proton counts.

The other options do not accurately define isotopes, as they incorrectly address the relationships between protons and neutrons or mischaracterize the properties of atoms.