Isotopes are atoms of a chemical element that have 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 has three isotopes: protium (one proton), deuterium (one proton and one neutron) and tritium (one proton and two neutrons).
In 2151, the Malurians were secretly mining the Akaali homeworld for veridium isotopes. (ENT: "Civilization")
In the mid-24th century, the Haakonians used Metreon isotopes in the metreon cascade, a weapon used on the Talaxian moon, Rinax. (VOY: "Jetrel")
Trilithium isotopes are used in Romulan plasma torpedoes. (DS9: "Image in the Sand")