One of the principal ways to classify matter is to categorize it as either an element or a compound. Elements, of which only around 118 have been identified, are some of the most basic substances known. Compounds are formed when elements are combined in specific proportions and arrangements. Silver, an element, can exist in its pure elemental form or as a compound combined with other elements. Typically, when silver exists as a compound, it does so by giving up electrons to other elements in the compound. When it does this, the silver atom becomes positively charged and the other elements become negatively charged. Charged atoms or groups of atoms are called ions and compounds containing ions are called ionic compounds or salts.
It is important to realize that when an element is transformed to an ion by giving up or accepting electrons, most of it's physical and chemical properties change too. A good example of this is when the two elements, sodium and chlorine, are combined to form the ionic compound sodium chloride. Sodium, in it's elemental form, is a soft, silvery metal that reacts violently with water, and chlorine is a suffocating, caustic, green gas. However, they combine to form the salt, sodium chloride, or common table salt. The differences in the physical and chemical characteristics between the elemental form and the combined, ionic, compound-form are dramatic.