Chemical bonds are formed when unstable, reactive atoms seek out stable configurations through sharing, donating or receiving valence electrons from other atoms. Bonds are broken down when a specific amount of energy known as the bond energy is applied to the bond.
The bond energy required to break down a bond is dependent on the type of bond. The stronger the bond, the greater amount of energy needed to destroy it. Because covalent bonds are strong, bonds do not spontaneously break down in nature without the application of bond energy. The bond energy can come from different energy sources, such as heat. Endothermic reactions are chemical reactions where heat is absorbed to apply the bond energy necessary to destroy the bond and change the compounds.
Chemical bonds do form spontaneously because atoms are naturally unstable and reactive. The type of bond they form depends on their electronegativity. Smaller differences in electronegativity result in covalent bonds and greater differences result in ionic bonds. The attraction and intermolecular forces of the bonds determine their strength and bond energy. Because chemical bonding involves the transfer of energy, such as atoms donating and receiving valence electrons in ionic bonds, it results in exothermic reactions, or reactions that give off heat energy.