All carbon atoms have 6 protons — that is what makes them carbon. But not all carbon atoms are identical. Carbon-12 has 6 neutrons, carbon-13 has 7, and carbon-14 has 8. These different versions of the same element are called isotopes, from the Greek words meaning "same place" — because they occupy the same position in the periodic table. Chemically, isotopes of an element behave almost identically (they form the same bonds and compounds), but their different masses give them distinct physical and nuclear properties.
The most famous isotope application is radiocarbon dating. Carbon-14 is radioactive, decaying with a half-life of about 5,730 years. Living organisms constantly absorb carbon from the environment, maintaining a steady ratio of C-14 to C-12. When an organism dies, it stops absorbing carbon, and the C-14 slowly decays. By measuring how much C-14 remains in an archaeological sample, scientists can determine when the organism died — a technique that revolutionized archaeology and earned Willard Libby the 1960 Nobel Prize. This method has been used to date everything from the Dead Sea Scrolls to prehistoric cave paintings.
Isotopes also play a vital role in medicine. Technetium-99m (the "m" stands for metastable) is the most widely used medical isotope in the world, employed in over 30 million diagnostic procedures annually. It emits gamma rays that cameras can detect, allowing doctors to image organs, bones, and blood flow without surgery. Other isotopes, like iodine-131, are used therapeutically to treat thyroid cancer. On the energy front, uranium-235 is the isotope that sustains nuclear chain reactions in power plants, while its more common sibling uranium-238 cannot. Same element, different neutrons, vastly different capabilities.