Isotopes
Versions of an element with the same number of protons but different numbers of neutrons; their ratios act as natural fingerprints.
Every atom of a given element has the same number of protons in its nucleus, but the number of neutrons can vary. These variants are isotopes. Carbon, for example, always has six protons, and comes as carbon-12 (six neutrons), carbon-13 (seven) and carbon-14 (eight). Isotopes of an element have the same arrangement of electrons, so they behave almost identically in chemical reactions. They differ mainly in mass, and some, like carbon-14, are radioactive.
That makes the ratios between isotopes powerful fingerprints. The proportions differ from place to place but are hardly changed by melting or ordinary chemistry. Some of the isotopes of lead are produced over time by the slow radioactive decay of uranium and thorium, so lead ores of different ages and origins end up with different mixtures. Measure the lead isotope ratios in a metal object and you can compare them with known mines. A 2023 study analysed 67 brass bracelets called manillas, traded on the West African coast, and found strong similarities with the metal of more than 700 Benin Bronzes; their chemistry pointed to the German Rhineland as the main source.
Oxygen isotope ratios in Moon rocks are nearly identical to Earth's, an important clue to how the Moon formed, while meteorites from different parent bodies carry their own signatures.
Radioactive isotopes decay at known rates, which is the basis of radiometric dating methods such as radiocarbon dating.
Coming up in future stories
- Corals Glow With the Floods of Centuries Past 1985 · Great Barrier Reef, Australia
- He Dated the Earth and Found Lead Everywhere 1948–1965 · Chicago and Pasadena, USA
- The Benin Bronzes Were Made of German Brass 16th–19th century · Benin City, Nigeria