Heterolysis can generate a (a positively charged carbon intermediate) or a (a negatively charged carbon species). Two sigma bonds are broken and a pi bond is formed between adjacent atoms in reactions. A one-step reaction is called a reaction, while a stepwise reaction involves more than one step, and the staring material is first converted to an unstable intermediate called a intermediate. A reaction replaces an atom or group of atoms with another atom or group of atoms. In an reaction, elements are added to a starting material. A mechanism is a detailed description of how bonds are broken and formed as a starting material is converted to a product. Homolysis generates reactive intermediates with unpaired electrons called radicals, while heterolysis generates intermediates.