Free radicals are atoms, molecules, or ions that contain at least one unpaired electron in their outer shell. Because electrons prefer to exist in pairs, this makes free radicals highly reactive and play an important role (pivotal) in organic synthesis, atmospheric chemistry, and biochemistry.
Most Read Free Radicals: Formation, Structure, Stability, Reactivity
Steps to Identify Free Radicals:
- Count Valence Electrons: Sum up the total number of valence electrons contributed by all atoms in the molecular formula.
- Check for an Odd Total: If the molecule has an odd total number of valence electrons, it is guaranteed to be a free radical (as at least one electron cannot be paired).
- Analyze the Structure (For Even Totals): If the total is even, it is usually a stable, paired molecule. However, check if it is a known diradical (like molecular oxygen, O2), where electrons remain unpaired in separate orbitals.
- Draw the Lewis Structure: Map out the bonds and lone pairs to visually locate the lone, unpaired electron dot (•).
Examples:
Methyl Radical (CH3•):
- Carbon contributes 4 valence electrons.
- Three Hydrogens contribute 3 electrons (1 each).
- Total = 7 valence electrons (Odd number).
- Verdict: Free radical with the unpaired electron centered on the carbon atom.
Hydroxyl Radical (•OH):
- Oxygen contributes 6 valence electrons.
- Hydrogen contributes 1 electron.
- Total = 7 valence electrons (Odd number).
- Verdict: Free radical with the unpaired electron centered on the oxygen atom.
Important Exception (Even Electron Radical):
Molecular Oxygen (O2): Each oxygen contributes 6 electrons (2s22p4) for a total of 12 (an even number). However, due to its molecular orbital structure, its ground state (triplet oxygen) contains two unpaired electrons, making it a diradical.
Takeaway:
An odd number of total valence electrons is a foolproof indicator of a free radical. For even-electron systems, a look at advanced molecular structures or Lewis exceptions is required to spot radical behavior.