Q. General Characteristics of the f-block?
You know that the 14 elements from cerium (Z = 58) to lutetium (Z = 71), which follow lanthanum (Z = 57) in the periodic table, are known as lanthanides, lanthanons or lanthanoids. Note that some authors include lanthanum also in lanthanides, but there IS no general agreement on it. These elements are characterised by successive filling of 4f orbitals in their atoms. These elements along with yttrium and lanthanum were originally known as rare earth elements or simply rare earths. The word 'earth' was used because they occur as oxides, which in early usage meant earth, and the word rare was used because of the great hard in their separation from each other. Otherwise, these are not particularly rare in earth's crust. For illustration, lanthanum, neodymium and cerium are more abundant than lead. Even the scarcest of them, thulium, is as abundant as bismuth and more abundant than arsenic, cadmium, selenium, or mercury none of which is usually considered rare.
The fourteen elements from thorium (Z = 90) to lawrencium (2 = 103) following actinium in the periodic table are define as actinides, actinoids or actions. They are analogous to the lanthanides and result from the filling of the of orbitals as the lanthanides result from the filling of 4f orbitals. Prior to 1940, only the obviously occurring actinides, i.e., thorium, protactinium and uranium were known. The remaining actinides have been produced artificially since then and are collectively define as transuranium elements.