Answer:
The answer is Synthetic elements.
Explanation:
Synthetic elements are those chemical elements that do not appear naturally on Earth, and can only be created artificially. These synthetic elements were not known to mankind until they were synthesized. They are radioactive elements, that is, unstable, with short half-lives compared to the age of the planet. Therefore they disintegrated almost completely since the formation of the Earth, and they are not found in appreciable quantities except for human action, produced in nuclear reactors or particle accelerators.
