A characteristic of electrons in a given orbital is that they demonstrate opposite spins. Within a molecule, two electrons sharing the same orbital exist in a bond called a covalent bond. A lone electron within an orbital is considered unpaired. A partial list of common naturally occurring free radicals is shown in the following table adopted from Halliwell. A Molecular Dr. Jekyll and Mr. Hyde Oxygen ushers in life.
Oxygen terminates life.
At a fundamental level, life is stored energy of carbon in its various reduced forms. Life is sustained by release of that energy as carbon-containing compounds are oxidized by oxygen to produce water and carbon dioxide. Diatomic oxygen in ambient air is considered a radical because it contains two unpaired electrons. This structural characteristic of oxygen, according to thermodynamics, should allow oxygen to cause immediate combustion of all organic molecules that come in contact with it. The explanation is that the two unpaired electrons of diatomic oxygen in two different orbitals have the same spin quantum number.