The Periodic System
Many attempts were made to classify the elements on the basis of various properties, such as the acidic or basic characteristics of their oxides, or their valence, or their metallic and nonmetallic qualities.
1. Basic and acidic oxides
Almost coincident with the development of a fairly clear distinction between acids and bases, the elements were separated into two large groups, according to whether their oxides produced acids or bases with water. These groups contained acid-forming and base-forming elements respectively. In this system a number of elements occur in both groups. For instance, manganese finds a place in each division because it yields a basic oxide, MnO, and an acidic oxide, Mn2O7. Furthermore, some oxides do not combine with water. Thus this broad classification is inadequate.
In spite of being considered as an classification property, valence is too much inconstant to be the basis of any systematic arrangement of the elements.
3. Metallic and nonmetallic elements
If an element in its solid or liquid state neither has a metallic luster nor conducts electricity, it is certainly a nonmetal; if it as the luster and is a conductor, it may be either a metal or a nonmetal. Because of a lack of agreement of physical properties, a consideration of chemical properties in classifying the elements into metals and nonmetals is in order. Typical metals lose electrons to form positive ions while nonmetals gain electrons to form negative ions. However, while some metals such as chromium and manganese act as expected in the formation of certain compounds, they may also constitute a part of negative ions! Thus no perfectly sharp distinction can be made between metallic and nonmetallic elements on this basis.