Review of Basic Concepts and Terminology

Many chemical species have a tendency to either give up or take on electrons. This tendency is based on the premise that a greater stability, or lower energy state, is achieved as a result of this electron donation or acceptance. A hypothetical example is the reaction between a sodium atom and a chlorine atom  

A sodium atom with only one electron in its outermost energy level would achieve a lower energy state if this electron were released to the chlorine atom, which would also achieve a lower energy state as a result. Both atoms would become ions (Na+ and Cl ), and each would have a stable, filled outermost energy level identical with those of the noble gases, neon in the case of sodium and argon in the case of chlorine. Thus, the electron transfer does take place and sodium chloride, NaCl, is formed. 

The reactions that this sodium-chlorine case typifies are called oxidation-reduction reactions. The term oxidation refers to the loss of electrons, while the term reduction refers to the gain of electrons. A number of oxidation-reduction reactions (nicknamed redox reactions) are useful in titrimetric analysis, and many are encountered in other analysis methods. 

It may appear strange that the term reduction is associated with a gaining process. Actually, the term reduction was coined as a result of what happens to the oxidation number of the element when the electron transfer takes place. The oxidation number of an element is a number representing the state of the element with respect to the number of electrons the element has given up, taken on, or contributed to a covalent bond. For example, pure sodium metal has neither given up, taken on, nor shared electrons, and thus its oxidation number is zero. In sodium chloride, however, the sodium has given up an electron and becomes a +1 charge thus its oxidation number is +1. The chlorine in NaCl has taken on an electron 

Halogen (group VIIA) in binary compound with a metal -1 or hydrogen 

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