They can affect the bonds between the atoms of the substrate, generating a constitutional isomer, or they can only involve an structural change, generating a stereoisomer. Both cases are generally clasified as isomerizations. When the energy difference between various structures is small, we have conformers (for example, alkanes, H2O2, hydrazine (NH2NH2) and it is understood that, at ordinary temperatures, are all in a dynamic equilibrium with ratios determined by the temperature and their relative stabilities. In this section only transformations between isomers will be described.