If you look at a chart of the periodic table, two rows of elements appear separate from the main table. These two rows comprise the lanthanide (or lanthanoid) group and the actinide (or actinoid) group.
The elements in these two rows are also known as the f-bock elements. This refers to the filling of the f-sub-shell by electrons. The f-sub-shell contains 14 electrons when filled. The lanthanide an actinide groups each contain 15 elements, as both also contain one element belonging to the d-block (containing a single electron in the d-sub-shell).
The lanthanide series of elements get their name from the first element of the series lanthanum (La). Lanthanum is followed by cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd) , terbium (Tb) dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and finally the d-block element lutetium (Lu). Their atomic numbers run from 57 (lanthanum) to 71 (lutetium). Although they have the name rare-earth elements most of the lanthanides are relatively common. Lanthanide metals are never found in a pure state in the natural world but are present in a number of mineral ores from which they are extracted by the ion exchange and solvent extraction methods.
Lanthanides are all soft silver/gray colored metals with high melting and boiling points. The freshly cut surface of the pure metal tarnishes as the element forms oxide with atmospheric oxygen. They also react with water releasing hydrogen. The hardness of the metals increases with the atomic number along the series. With dilute acids, lanthanide reactions result in the rapid evolution of hydrogen gas. Reactions of lanthanide metals with other elements are exothermic, particularly those reactions involving hydrogen.
Members of the lanthanide series form ionic compounds by the loss of one or more electrons some of these compounds are pale colored. Some compounds exhibit paramagnetic properties and some fluoresce on exposure to ultraviolet light.
The 3+ ions, formed by donating three electrons, exhibit the “lanthanide contraction”. In this, the atomic radius of the 3+ ions decreases as the atomic number increases.
The actinide series also gets its name from the first member of the series, the element actinium (Ac). Within the series Actinium is followed by thorium (Th), protactinium (Pa), uranium (U), neptunium (Np), plutonium (Pu), americium (Am), curium (Cu), berkelium (Bk), californium (Cf), einsteinium (Es), fermium (Fm), mendelevium (Md), nobelium (No) and the d-bock element lawrencium (Lr). Uranium (atomic number 92) is the heaviest naturally occurring element, all elements above this in the periodic table are manmade.
All of the actinides are radioactive. Actinide elements are electropositive combining readily with non-metallic elements. They are dense metals, which tarnish in air. Reacting less vigorously than lanthanides, they release hydrogen from dilute acids and boiling water.
Los Alamos National Laboratory Chemistry Division
Jefferson Laboratories Science Education website