The isotopes of an element have the same number of protons in their atoms (atomic number) but different masses due to different numbers of neutrons.In an atom in the neutral state, the number of external electrons also equals the atomic number.
Stable isotopes are tools used by researchers worldwide in the diagnosis of disease, to understand metabolic pathways in humans, and to answer fundamental questions in nature.
Not all the atoms of an element need have the same number of neutrons in their nuclei. Three nuclei with one proton are known that contain 0, 1, and 2 neutrons, respectively.
In fact, it is precisely the variation in the number of neutrons in the nuclei of atoms that gives rise to isotopes. The three share the place in the periodic table assigned to atomic number 1 and hence are called isotopes (from the Greek isos, meaning "same," and topos, signifying "place") of hydrogen.
The numerical difference between the actual measured mass of an isotope and A is called the mass defect.
The specification of Z, A, and the chemical symbol (a one- or two-letter abbreviation of the element's name, say Sy) in the form A/ZSy identifies an isotope adequately for most purposes.