According to the modern view of chemical reactions, bonds between atoms in the reactants must be broken, and the atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds, and energy is evolved as bonds are made. In some reactions the energy required to break bonds is larger than the energy evolved in making new bonds, and the net result is the absorption of energy. Hence, different types of bonds may be formed in a reaction. A Lewis acid-base reaction, for example, involves the formation of a covalent bond between a Lewis base, a species that supplies an electron pair, and a Lewis acid, a species that can accept an electron pair. Ammonia is an example of a Lewis base. A pair of electrons located on a nitrogen atom may be used to form a chemical bond to a Lewis acid.