The f block elements are formed of two series the Lanthanoids (or Lanthanide series) and the Actinoids (or Actinide series). They are also sometimes called rare earth elements, although many are more abundant than the platinum group of elements. Most elements are grouped vertically in the periodic table but the f block elements are grouped horizontally and have consecutive atomic numbers. The Lanthanoids are in the sixth period and the Actinoids in the seventh period of the table.
The term f-block refers to the filling of the f electron shell. Lanthanoids fill the 4f electron shell while Actinoids fill the 5f shell.
Lanthanoids consists of 15 elements: Lanthanum (atomic number 57), Cerium (58), Praseodymium (59), Neodymium (60), Promethium (61), Samarium (62), Europium (63), Gadolinium (64), Terbium (65), Dysprosium (66), Holmium (67), Erbium (68), Thulium (69), Ytterbium (70), and Lutetium (71). All except Lutetium are f block elements; Lutetium is a d block element. The elements have not been found in their pure state in nature but can be found in mineral ores such as monazite sands and gadolinite.
The Lantanoids are fairly soft metals their silvery-white appearance tarnishes in are as they form oxides. As the atomic number increases so does the hardness of the metallic element. The radius of the 3+ ion decreases as the atomic number increases this phenomenon is known as the "lanthanide contraction". The elements are highly reactive and will react with water to release hydrogen. With dilute acids this hydrogen release is very rapid. The metals have high melting and boiling points. Some can ignite and burn vigorously when subjected to high temperatures. This tendency towards exothermic reactions is particularly noticeable in the reaction between Lanthanoids and hydrogen.
Lanthanoids usually form ionic compounds which tend to be pale colored. Most of the compounds formed are paramagnetic and many will fluoresce when expose to ultra violet light.
The fifteen elements making up the Actinoid series are Actinium (89), Thorium (90), Protactinium (91), Uranium (92), Neptunium (93), Plutonium (94), Americium (95), Curium (96), Berkelium (97), Californium (98), Einsteinium (99), Fermium (100), Mendelevium (101), Nobelium (102), and Lawrencium (103). Lawrencium is a d block Actinoid while all the others are f block elements. All the elements above Uranium are man made and are sometimes called trans-uranium elements.
The Actinoids are all radioactive. They are dense metals, which will tarnish easily in air. They may have a number of allotropes at least six allotropes have been recognized for the element plutonium. They are capable of reacting with dilute acids or boiling water to release hydrogen gas. Actinoid elements are highly electropositive and will combine readily with most non-metallic elements.