A reaction mechanism is a step-by-step description of how a chemical reaction takes place, showing the movement of electrons and the formation of intermediate species.

Heterolytic fission involves one (more electronegative) atom taking both electrons from a covalent bond, forming a positive ion and a negative ion.

Homolytic fission results in each bonded atom taking one electron, forming two radicals, each with an unpaired electron.

A free radical is a species with an unpaired electron (represented by a dot).

The unpaired electron in free radicals makes them very reactive.

The movement an electron pair, showing how a bond is broken or formed during a chemical reaction.

A carbocation is an ion that contains a positively charged carbon atom.

For example, the structure of the C(CH₃)H₂⁺ carbocation is:

An electrophile is an electron-pair acceptor.

An electrophile is an electron-deficient species that accepts a pair of electrons to form a new covalent bond. Examples of electrophiles include AlCl₃, Br⁺ and H₃O⁺.

An addition reaction is a reaction in which two reactants combine to form one larger product.

C₂H₄ + Br₂ ➔ C₂H₄Br₂ is an example of an addition reaction.

An elimination reaction involves the removal of a small molecule from a larger one, resulting in the formation of two products.

C₂H₅OH ➔ C₂H₄ + H₂O is an example of an elimination reaction.

In a substitution reaction, an atom or group of atoms is replaced by a different atom or group of atoms in a molecule.

CH₄ + Cl₂ ➔ CH₃Cl + HCl is an example of a substitution reaction.

[O] represents an oxygen atom from an oxidising agent.

For example, C₂H₅OH + 2[O] ➔ CH₃COOH + H₂O represents the oxidation of ethanol.

A nucleophile is an electron-pair donator.

A nucleophile is an electron-rich species that donates a pair of electrons in a reaction. Examples of nucleophiles include OH^-, CN^- and Br^-.

[H] represents a hydrogen atom from an reducing agent.

For example, CH₃COCH₃ + 2[H] ➔ CH₃CH(OH)CH₃ represents the reduction of propanone.