What is unstable, highly reactive atoms or molecules…that can cause internal body damage?

Highly reactive atoms or molecules that can cause damage to cells are called free radicals.
Free radical is an atom, molecule, or ion that has an unpaired valence electron (electron in outermost shell). Because of that, free radicals are very unstable and they react quickly with other substances in order to make up the number of electrons in their outer shell. Free radicals may be generated in many different ways such as redox reactions, ionizing radiation, heat, electrical discharges..
Oxidative stress is a process which happens within the body and it is caused by oxygen free radicals or reactive oxygen species (ROS). Oxidative stress can damage cells, leading to a range of diseases and cause aging.