Periodic breathing: Clusters of breaths separated by intervals of apnea (no breathing) or near-apnea. As opposed to normal breathing which is usually regular. Periodic breathing was originally thought to arise from serious neurologic or cardiovascular disease and therefore to carry a poor outlook.

Interactive periodic table with element scarcity (SRI), discovery dates, melting and boiling points, group, block and period information.

The periodic table has been used by chemists to observe patterns and relationships between elements. There are 3 main groups in the Periodic Table; metals , metalloids , and nonmetals . For example, elements to the bottom and far left of the table are the most metallic , and elements on the top right are the least metallic .