Isotope
One of two or more atoms of the same element that differ in neutron number and therefore in mass.
Formula / equation
same Z, different A
Unit
Dimensionless
In depth
Isotopes share an identical electron configuration, so their chemistry is essentially the same; only mass-dependent properties such as diffusion rate, vibrational frequency and reaction rate differ measurably. This kinetic isotope effect is exploited to trace reaction mechanisms, while isotope ratios date rocks and reveal the origin of samples.
Examples in the real world
Hydrogen's protium ¹H, deuterium ²H and tritium ³H; the ³⁵Cl and ³⁷Cl mixture that gives chlorine its 35.45 average mass.