Think of them as mutant but harmless atoms of a single element that have different masses. The common isotope of hydrogen has one proton and one electron. But the hydrogen isotope deuterium also has a neutron, and is almost twice as heavy as the common hydrogen isotope. With a mass spectrometer, of course. That difference in behavior gives scientists an opening big enough to drive a mass spectrometer through.
Radioactive isotopes change over time into other isotopes.
Geologists have been analyzing stable isotopes for decades. Lately, climatologists have joined in the fun. Now it's the biologists' turn. . Monarchs dining on milkweed nectar. Photo by Karen Oberhauser.