Isotope

Isotopes are atoms of the same element with different numbers of Neutrons in the Atomic nucleus. Same Proton count, same chemistry, different mass — and often very different stability.

Familiar families

element	isotope	neutrons	character
hydrogen	¹H (protium)	0	stable — most of the universe
hydrogen	²H (deuterium)	1	stable — heavy water
hydrogen	³H (tritium)	2	radioactive, ~12 years
carbon	¹²C	6	stable — 98.9% of carbon
carbon	¹⁴C	8	radioactive, half-life ~5,700 years
uranium	²³⁵U	143	fissile — reactor fuel
uranium	²³⁸U	146	99.3% of natural uranium

What isotopes are for

• Radiocarbon dating — living things take in ¹⁴C; when they die the clock starts. The ~5,700-year half-life dates anything organic back tens of thousands of years.
• Medicine — short-lived tracers light up PET scanners through Positron emission.
• Nuclear fuel — enrichment is nothing but sorting uranium isotopes.

The word was coined by Frederick Soddy in 1913 — Greek for "same place," because the variants share one spot in the periodic table. It earned him the 1921 Nobel Prize in Chemistry.