Isotopes
Variants of a particular chemical element that possess identical atomic numbers Z (same number of protons) but different mass numbers A (different number of neutrons).
Governing formula
Nuclide Symbol: ᴬ_Z X (Same Z, Different A)
SI unit
Dimensionless Species
In depth
Because isotopes have identical proton numbers and electron shell configurations, they exhibit virtually identical chemical behavior, but differ in mass, nuclear spin, and radioactive decay stability.
Examples in the real world
Protium (¹H), Deuterium (²H), and radioactive Tritium (³H) serving as isotopes of hydrogen.