Period 6 element between barium and hafnium, and radium and rutherfordium, respectively. This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table inserts the lanthanide and actinide series in their proper places, as parts of the table's sixth and seventh rows (periods). Lanthanum is a chemical element with the symbol La and atomic number 57. It is the first element in the lanthanide series. It is a soft, ductile, silvery-white metal that tarnishes rapidly when exposed to air and is soft enough to be cut with a knife. It is the eponym of the lanthanide series,
Non-metallic inclusions Non-metallic inclusions Non-metallic inclusions are chemical compounds and nonmetals that are present in steel and alloys. They are the product of chemical reactions, physical effects, and contamination that occurs during the melting and pouring process. These inclusions are categorized by origin as either endogenous or exogenous. Endogenous inclusions, also known as indigenous, occur within the metal and are the result of chemical reactions. These products precipitate during cooling and are typically very small. Exogenous inclusions are caused by the entrapment of nonmetals. Their size varies greatly and their source can include slag, dross, flux residues, and pieces of the mold.