A lanthanide is one of a group of 15 elements that have atomic numbers 57 to 71. This is also known as the lanthanide series or lanthanoid series. This group starts at lanthanum and it ends at lutetium. All lanthanoids are f-block elements, meaning that electrons fill the 4f electron shell, except for lutetium which is a d-block lanthanoid. The lanthanoid series (Ln) is named after lanthanum.