= Addition reaction
An <addition reaction> joins reacting <molecular entities> while reducing bond multiplicity and retaining their atoms in the combined product. For example, adding <hydrogen> across an <alkene>'s carbon–carbon double <chemical bond> produces a saturated product. The reverse transformation is an <elimination reaction>.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[Addition reaction, entry A00133]{ref}
The net transformation does not uniquely specify the mechanism. Electrophilic, radical and concerted routes can give addition products with different rate laws and <stereochemistry>. Tracking both the atoms and the changing <bond order> separates an <addition reaction> from a replacement or rearrangement.
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