Radicals (DP IB Chemistry): Revision Note

Radicals

• Reaction equations show the amounts of reactants and products, including stoichiometry

• Reaction mechanisms explain how the reaction happens, involving electron movement and different chemical species

• One key type of species involved in mechanisms is a radical

What are radicals?

• A radical is a chemical species that has an unpaired electron 

• They can be described as:

  • Atomic - a single atom with an unpaired electron

  • Polyatomic / molecular - a group of atoms bonded together with no overall charge, that contains an unpaired electron

  • Anionic - an atom or molecule that gains one electron to become an anion AND has one atom with an unpaired electron 

  • Cationic - an atom or molecule that loses one electron to become a cation AND has one atom with an unpaired electron

• The sole requirement for a radical is the unpaired electron

  • They can exist independent of the charge on the chemical species, e.g. cations must have a corresponding anion and vice versa 

Examples of radicals table

• Radicals are indicated by the dot (•) in the formula of the chemical species

  • When the radical is made of several atoms, the radical dot should be shown on the atom with the unpaired electron

  • This is most obviously seen in displayed formulae

The methyl radical

Reactivity of radicals

• Radicals are highly reactive due to their unpaired electron

• This unpaired electron gives them high enthalpy

• To lower their enthalpy, radicals tend to react quickly by:

  • Stealing an electron from another species (which then becomes a new radical)

  • Combining with another radical to form a covalent bond

  • Because of this high reactivity, radicals are usually short-lived