FTP, name this effect caused by the properties of f orbitals, and named for a group of elements starting with element 57.
It is caused by strange orbital angular geometries, which makes their electrons poor at schielding the charge of the nucleus and increases effective nuclear charge at a rate much higher than the general trend.  It explains a number of unexpected properties of certain elements, such as why molybdenum and tugsten have nearly the same atomic radius despite tungsten's having 32 more electrons, and also the extreme physical and chemical similarity of many of its namesake elements, which include the bunch discovered in Ytterby, Sweden.  FTP, name this effect caused by the properties of f orbitals, and named for a group of elements starting with element 57.
It is caused by strange orbital angular geometries, which makes their electrons poor at schielding the charge of the nucleus and increases effective nuclear charge at a rate much higher than the general trend.  
lanthanide contraction