The periodic table - classification of elements. Definitions of groups, periods, alkali metals, alkaline earth metals, halogens, and noble gases. How metals, non-metals, and metalloids can be identified by the position on the periodic table. Created by Jay. Introduction to the periodic table. The periodic table - classification of elements. Groups of the periodic table. Counting valence electrons for main group elements.
OPENBook is software developed for blind and visually impaired individuals to read, edit, and manage scanned images from books, magazines, manuals, bills, newspapers, and other printed documents. It essentially turns your computer system into a scanning and reading machine.
In object-oriented programming, the open/closed principle states  software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification ; that is, such an entity can allow its behaviour to be extended without modifying its source code.