Isotopes were atoms of a chemical element that had the same atomic number (making them essentially the same element) but different mass numbers (resulting in different properties) due to a different number of neutrons in the atom's nucleus. For example, hydrogen had three isotopes: protium (one proton), deuterium (one proton and one neutron) and tritium (one proton and two neutrons).
A gas giant investigated by the Enterprise in 2152 had many moons with atmospheres containing selenium isotopes. These isotopes interfered with sensors, hampering the search for Charles Tucker when his shuttlepod was shot down over one of the moons. (ENT: "Dawn")