A radical is a highly reactive species which has one unpaired electron. They tend to be formed by homolytic fission of single bonds - this is in contrast to the heterolytic fission which dominates the chemistry of nucleophiles and electrophile - as shown below;

In the gas phase, this bond-breaking is always more energetically favourable than the alternative heterolytic fission to produce cation and anion.

However, in polar solvents particularly, the energetic stabilisation of the ions (solvation enthalpy) makes heterolytic fission the more favoured. Hence many reactions which occur in the gas phase will be radical reactions but some may also occur in solution, especially if the solvent is non-polar and the homolytic fission can be initiated.

What this means is that some energy must go into the system to homolytically cleave the bond and create two radical species - this energy is usually in the form of heat (abbreviated in reaction schemes as 'Δ') or light (abbreviated 'hν').

This energy will split apart a weak single bond - some examples of weak single bonds are given here, note that they tend to be between heteroatoms, where the bond is weakened by mutual repulsion between nonbonded electrons.

A very important feature of radical reactions is the fact that they proceed as chain reactions - where the reaction keeps itself going until all is finished. The key to chain reactions is the three parts - initiation, propagation and termination. They can be summarized as follows;

Initiation: The creation of a pair of radicals from a non-radical species - e.g.;

Propagation: The reaction of a radical with a non-radical species, to create a product, plus a radical which continues the chain - e.g.;

Termination: The combination of any two radical species - leaving no radicals to continue chain reacting - e.g.;

Because they are so reactive, radicals are rarely actually trapped and observed - so they are often termed reactive intermediates. The reactivity of radicals means that they are less selective than nucleophiles and electrophiles about where they attack a molecule, so this can make them slightly uncontrollable in some circumstances - however many synthetic uses of radicals exist.