Isotope
Atoms of the same element (same atomic number Z) that have different numbers of neutrons and therefore different mass numbers (A). Isotopes of an element are chemically similar because they have the same electron configuration but differ in nuclear mass and radioactive stability. Common examples include carbon-12 and carbon-13, or uranium-235 and uranium-238.
Real World
Uranium-235 is the isotope used as fuel in nuclear power stations like Hinkley Point C, while the more abundant uranium-238 is not readily fissile, despite both being uranium (Z = 92).
Exam Focus
Define isotopes as 'same number of protons, different number of neutrons' — never say 'same element, different mass' alone.
How well did you know this?