Valence refers to a number assigned to elements that reflects their ability to react with other elements and the type of reactions the element will undergo. The term valence is derived from the Latin word for strength and can reflect an element's strength or affinity for certain types of reactions.

The electrons in an atom are located in different energy levels. The electrons in the highest energy level are called valence electrons. In accord with the octet rule--and to become more energetically stable--atoms gain, lose, or share valence electrons in an effort to obtain a noble gas configuration in their outer shell. The configuration of electrons in an atom's outer shell determines its ability and affinity to enter into chemical reactions.

The valence number of an element can be determined by using a few simple rules relating to an element's location on the periodic table. In ionic compounds...