>Complete Electron Transfer >A reduction-oxidation (redox) reaction always involves two events: one substance loses electrons and is said to be oxidized, while another substance gains electrons and is said to be reduced. >Sodium chloride, or table salt, offers an example of atoms that have undergone oxidation and reduction to become ions. >When sodium and chlorine react to form sodium chloride, one electron is completely transferred from the sodium atom to the chlorine atom. >The result is a sodium cation and a chloride anion. >In this redox reaction, the sodium atom lost an electron - it was oxidized. >The new sodium ion is a positively charged cation. >The chlorine atom gained an electron - it was reduced. >The new chloride ion is a negatively charged anion. >Electron Sharing >In redox reactions, electrons can be transferred completely from one molecule or atom to another, forming an ionic bond, or they can simply shift positions in covalent bonds. >The combustion of methane is a good example of this second type of reaction. >The electrons in the two reactants - methane and oxygen - are shown as dots at equal distance from the atomsâ nuclei, indicating that electrons are shared equally. >When electrons are shared equally, the bonds are nonpolar. >Methane combustion involves the reaction of two oxygen molecules with each methane molecule. >The reaction yields carbon dioxide and water as the products. >The atoms in the products share electrons unequally, meaning that the bonds are polar. >In carbon dioxide, the nucleus of the carbon atom holds the electrons less tightly. >Carbon has been oxidized - it âlostâ electrons. >In water, the nucleus of the oxygen atom holds the electrons more tightly. >Oxygen has been reduced - it "gained" electrons. >Because the atomic nuclei of the product molecules hold the electrons more tightly than they were held in the reactant molecules, the products have lower potential energy. >Such reactions tend to be spontaneous, or exergonic, and release free energy.