Oxidation-reduction is a formal system for our convenience. In this system, oxidation is defined as a process by which oxidation number increases, corresponding to a loss of electron control. In reduction oxidation number decreases, corresponding to a gain of electron control. In order to assign electron control across covalent bonds, it is essential to know which atoms are more electronegative in the bonds. How electronegativity differences play out in chemical reactions provides the underlying coherence of the oxidation-reduction system of chemistry. Electronegativity reflects the strength of attraction an atom has for the electrons it shares in chemical bonds, while oxidation-reduction is a systematic accounting procedure to reflect the changes in the bonding environment of electrons between products and reagents. When two atoms form a covalent bond, the more electronegative atom is assigned 'electron control' in the oxidation-reduction system. If an atom gains electron control through a chemical process, it is said to be 'reduced,' while the atom that has lost electron control is said to be 'oxidized'. The key to the system is that when a very electronegative atom is reduced, it draws the new electrons inwards towards its strongly attracting nucleus, and the bond becomes polarized. This closing of separation between unlike charges represents a potential energy decrease above and beyond the typical energy decrease that accompanying the formation of an ordinary bonding, molecular orbital. In other words, the formation of polar bonds corresponds to large potential energy decreases (tending toward negative internal energy change, negative enthalpy, negative free energy change), and as a general rule, polar bonds are stronger than nonpolar bonds (more energy is required to break them because the electrons have to be wrenched away from the oxidant). Oxidation-reduction provides a systematic way to account for these tendencies. The more electronegative elements form stronger covalent bonds.
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