What is an isotope, and why do isotopes matter?
Atoms of the same element with different numbers of neutrons — so they share chemical behaviour but differ in mass and, crucially, in nuclear stability.
The distinction that defines it. An element is defined by its number of protons: every carbon atom has six. Neutrons vary. Carbon-12 has six neutrons, carbon-13 has seven, carbon-14 has eight. All three are carbon and behave almost identically in chemical reactions, because chemistry is governed by electrons, which follow protons.
Stable and radioactive. Some isotopes are stable indefinitely; others have nuclei that are energetically unfavourable and decay, emitting radiation and transforming into other isotopes or elements. Each radioactive isotope has a characteristic half-life, ranging from fractions of a second to billions of years.
Why they are genuinely useful:
Dating. Radioactive decay is a clock. Carbon-14 dates organic material over tens of thousands of years; uranium-lead dates rocks over billions, which is how the age of the Earth was established.
Medical imaging and treatment. Short-lived isotopes injected as tracers reveal metabolic activity — this is how PET scanning works — and targeted isotopes deliver radiation to tumours.
Tracing in chemistry and biology. Because isotopes behave chemically alike, a labelled atom can be followed through a reaction or a metabolic pathway, which is how many biochemical processes were mapped.
Climate and environmental reconstruction. Ratios of oxygen isotopes in ice cores and shells record past temperatures, because evaporation slightly favours the lighter isotope — so the ratio encodes climate history.
Provenance. Isotope ratios vary geographically, so they can indicate where food, drugs or a person originated.
Nuclear applications. Only specific isotopes are fissile, which is why enrichment — increasing the proportion of uranium-235 — is the central technical and political problem in nuclear power and weapons.
Why atomic masses are not whole numbers. A periodic table value is the weighted average of an element's naturally occurring isotopes, which is why chlorine appears as about 35.5.