Noble gases generally do not readily form chemical bonds with other elements. Combined with the similarity in chemical properties among all elements that have the same number of valence electrons, the inert nature of the noble gases leads us to what is commonly called the octet rule . Even though there are some exceptions, the octet rule is an effective tool in predicting and explaining most of the combinations of atoms that occur during chemical reactions. Positive ions are called cations , and negative ions are called anions . The number of valence electrons in an atom of an element, in general, determines how many electrons will be transferred when an ion is formed and, therefore, what the charge on each ion will be in units of the fundamental electron charge.
Some metals are capable of forming ions with more than one electrical charge.
To avoid such confusion, the Stock system of using a Roman numeral to indicate the metal's ionic charge in a given compound was developed. Ionic compounds always form in such a way that the resulting formula units have an overall neutral charge. This idea will be helpful when we learn to write and balance equations for chemical reactions. The atoms of each reacting element that makes up a compound must combine in such a way that the total electrical charge of the formula unit formed is zero and all atoms must be isoelectronic with a noble gas. Lewis symbols are a convenient way to represent the valence shell electrons of atoms.