Isotope
One of two or more forms of the same chemical element whose atoms have different numbers of neutrons, and so different masses.
Every atom of a given element has the same number of protons in its nucleus. Oxygen always has 8, for instance. But the number of neutrons can vary, and each version is an isotope. Oxygen has three stable isotopes, oxygen-16, oxygen-17 and oxygen-18, where the number is the total of protons plus neutrons. Isotopes of one element behave almost identically in chemical reactions, but their slightly different masses can be measured with a mass spectrometer.
Some isotopes are stable and never change; others are radioactive and decay into different elements, which makes them natural clocks for dating. Stable isotopes are just as useful in another way, acting like fingerprints. Different bodies in the Solar System have slightly different mixtures of oxygen isotopes, so measuring them can show which meteorites came from the same parent body.
The HED meteorites, a family including the howardites, eucrites and diogenites, share an almost identical oxygen isotope fingerprint, one of the main reasons scientists think they come from one parent body, most likely the asteroid Vesta. Martian meteorites such as Nakhla share a different one. In writing, an isotope is shown by the element name and mass number, as in carbon-14 or oxygen-17.
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Level: Explorer Ages 10 and up