A free radical is an atom, molecule, or compound that is highly unstable because of its atomic or molecular structure (i.e., the distribution of electrons within the molecule). This instability makes free radicals very reactive, and they attempt to pair up with other molecules, atoms, or even individual electrons to create a stable compound. To achieve a more stable state, free radicals can “steal” a hydrogen atom from another molecule, bind to another molecule, or interact in various ways with other free radicals. Free radicals can be deﬁned as reactive chemical species having a single unpaired electron in an outer orbit and are continuously produced by the organism’s normal use of oxygen. This unstable conﬁguration creates energy that is released upon reaction with adjacent molecules, such as proteins, lipids, carbohydrates, and nucleic acids. The majority of free radicals that damage biological systems are derived from oxygen and more generally referred to as “reactive oxygen species.” There are many types of radicals, but those of most concern in biological systems are derived from oxygen, and known collectively as reactive oxygen species. Oxygen has two unpaired electrons in separate orbitals in its outer shell. This electronic structure makes oxygen especially susceptible to radical formation. Sequential reduction of molecular oxygen (equivalent to sequential addition of electrons) leads to formation of a group of reactive oxygen species:superoxide anion
peroxide (hydrogen peroxide)
Free radicals are any atom with an unpaired electron.
Reactive oxygen species are all the free radicals that are commonly found in biology that involve oxygen.
So ROS’s are free radicals, specifically involving oxygen.