The lanthanides and actinides form a group that appears almost disconnected from the rest of the periodic table. This is the f block of elements, known as the "inner transition series," so termed because if placed in its proper numerical position in the table it would be in the transition metals between groups 2 and 3.
The 14 elements (numbers 58 to 71) of the lanthanide series are also known as the rare earth elements. Most lanthanides are formed when uranium and plutonium undergo nuclear reactions. The f sub-level contains seven orbitals, each of which will hold two electrons. Therefore, it is possible to place 14 electrons in the 4f sub-level. Generally speaking, the lanthanides have electron configurations that follow the Aufbau rule, and the 4f sublevel is filled as atomic number increases from cerium (Ce) to lutetium (Lu). However, there are three lanthanide metals that have properties similar to the d block: cerium (Ce), lutetium (Lu), and gadolinium (Gd). All of these metals contain a d electron in their electron configuration.
A similar overall trend holds for the 14 elements in the actinide series (numbers 90 to 103): from thorium (Th) to Lawrencium (Lr), the 5f sublevel is progressively filled.
The chemistry of the lanthanides differs from main group elements and transition metals because of the nature of the 4f orbitals. These orbitals are "buried" inside the atom and are shielded from the atom's environment by the 4d and 5p electrons. As a consequence of this, the chemistry of the elements is largely determined by their size, which decreases gradually with increasing atomic number, from 102 pm (La3+) to 86 pm (Lu3+). This phenomenon is known as the lanthanide contraction. All the lanthanide elements exhibit the oxidation state +3. In addition, Ce3+ can lose its single f electron to form Ce4+, which has the stable electronic configuration of xenon. Similarly, Eu3+ can gain an electron to form Eu2+, which has the extra stability of the half-filled f7 configuration. Promethium is effectively a man-made element, as all its isotopes are radioactive with half-lives shorter than 20 y.
Actinides are typical metals. All of them are soft, have a silvery color (but tarnish in air), and have relatively high density and plasticity. Some of them can be cut with a knife. The hardness of thorium is similar to that of soft steel, so heated pure thorium can be rolled in sheets and pulled into wire. Thorium is nearly half as dense as uranium and plutonium but is harder than both of them.
Unlike the lanthanides, most elements of the actinide series have the same properties as the d block. Members of the actinide series can lose multiple electrons to form a variety of different ions. All actinides are radioactive, paramagnetic, and, with the exception of actinium, have several crystalline phases: plutonium has seven, and uranium, neptunium, and californium have three. Crystal structures of protactinium, uranium, neptunium, and plutonium do not have clear analogs among the lanthanides and are more similar to those of the 3d transition metals. All actinides are pyrophoric, especially when finely divided; that is to say, they spontaneously ignite upon exposure to air.
The melting point of actinides does not have a clear dependence on the number of f electrons. The unusually low melting point of neptunium and plutonium (~640 °C) is explained by hybridization of 5f and 6d orbitals and the formation of directional bonds in these metals. Like the lanthanides, all actinides are highly reactive with halogens and chalcogens; however, the actinides react more easily. Actinides, especially those with a small number of 5f electrons, are prone to hybridization. This is explained by the similarity of the electron energies at the 5f, 7s, and 6d shells. Most actinides exhibit a larger variety of valence states, and the most stable are +6 for uranium, +5 for protactinium and neptunium, +4 for thorium and plutonium, and +3 for actinium and other actinides.