The periodic table lists elements discovered or prepared in the laboratory in the order of their increasing atomic numbers. This enables to us place the elements under appropriate groups and thus predict the possible values of physical (like boiling point, melting point, etc) and chemical (valence electrons, possible bonds, etc) quantities. Currently, there are about 120 known elements. Some of these are artificially prepared in the laboratory and some are extremely reacting, hardly being able to exist under normal conditions.