Isotope
Definition
Atoms of the same element with different numbers of neutrons. They behave the same chemically but can differ in mass and stability.
Technical definition
Isotopes are atoms of the same element, meaning an identical atomic number, that differ in neutron count and therefore mass number. They share nearly identical chemical properties but may differ in physical properties and nuclear stability, with some being radioactive.
Example
Chlorine exists naturally as about 75% chlorine-35 and 25% chlorine-37. That is why its atomic weight is 35.45, sitting between the two.
Common confusion
Isotopes are not different elements or ions. Same protons means the same element and neutral charge; only the neutron count changes.