Three things are necessary to make a free radical. A stable molecule, oxygen which love to steal electron from a stable molecule and a metal or calcium atom. When oxygen approaches a molecule, it tries to steal the molecule’s electron. This is not happen readily if oxygen all by itself.
However, if an electron conducting metal ion or calcium atom is nearby, the metal or calcium will act as a catalyst to strip off the electron from the stable molecule. Now the oxygen is an unstable free radical with an extra electron and the molecule also become an unstable free radical. Inside our body, if calcium or metal such as lead or iron begin to accumulate in cells, the cells suffer massive amount of free radical damage or oxidation. Eventually resulting in diseases of the aging process. This is because metals and calcium are free radical catalyst.