Despite their name, they 
Despite their name, they occur widely. Because their f orbital electrons are not effective at shielding their outermost electrons, they give rise to a namesake contraction in atomic radius going across the Periodic Table, which explains why zirconium and hafnium are so similar. Their 4f electrons do not participate in bonding, and this group of elements includes holmium, samarium, and gadolinium. Also known as the rare earth elements, FTP, identify this group that starts with cerium and ends with lutetium, following its namesake element on the Periodic Table.
Despite their name, they occur widely. Because their f orbital electrons are not effective at shielding their outermost electrons, they give rise to a namesake contraction in atomic radius going across the Periodic Table, which explains why zirconium and hafnium are so similar.
{lanthanides} or {lanthanoids} or {lanthanons} (accept {rare earth} elements on early buzz)