Lanthanide

This series is the row above the actinide series, which is located underneath the main body of the periodic table. The lanthanides are often referred to as Rare Earth Metals. The members of this series are lanthanum (La), cerium (Cr), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), lutetium (Lu). They are called lanthanides because the elements in the series are chemically similar to lanthanum. Roughly 0.0018% of the Earth's crust is composed of lnthanium (La). Today, lanthanum is primarily obtained thorough an ion exchange process from monozite sand. The estimated crustal abundance is 3.9x101mg/kg, and the number of stable isotopes is 1. The lanthanides were first discovered in 1787 when an unusual black mineral was found in Ytterby, Sweden. This mineral, now known as gadolinite, was later separated into to various lanthanides elements. Today, the three main mineral sources are: monazite (contains mostly the lighter lanthanides), xenotime (contains mostly the heavier lanthanides) and euxenite (contains a fairly even distribution of lanthanides). Three of lanthanide elements have radioactive isotopes with long half-lives (138La, 147Sm and 176Lu) that can be used to date minerals and rocks from Earth, Moon and meteorites).

Like any other series in periodic table the Lanthanides share many similar characteristics such as:

• Similarity in physical properties throughout the series.
• Adoption mainly of the +3 oxidation state, usually found in crystalline compounds.
• They can also have an oxidation state of +2 or +4, though some lanthanides are most stable in the +3 oxidation state.
• Adoption of coordination numbers greater than 6 (usually 8-9) in compounds.
• Tendency to decreasing coordination number across the series.
• A preference for more electronegative elements (such as O of F) binding.
• Very small crystal-field effects.
• Little dependence of ligands.
• Ionic complexes undergo rapid ligand exchange.

The lanthanides have similarities in their electronic configuration, which explains most of the physical similarities. These elements have the electrons in the f orbital. After lanthanum, the energy of the 4f sub-shell falls below that of the 5d sub-shell. This means that the electrons start to fill the 4f sub-shell before the 5d sub-shell. Another important feature of the lanthanides is the lanthanide contraction, in which the 5s and 5p orbitals penetrate 4f sub-shell. This means that the 4f orbital is not shielded from the increasing nuclear change, which causes the atomic radius of the atom to decrease that continues throughout the series.

These metals have a silvery shine when freshly cut. However, they can tarnish quickly in air, especially Ce, La and Eu. These elements react with water slowly in cold, though that reaction can happen quickly when heated. This is due to their electropositive nature. The lanthanides have the following reactions:

• Oxidizing rapidly in moist air.
• Dissolve quickly in acids.
• Reaction with oxygen is slow at room temperature, but they can ignite around 150-200ºC.
• React with halogens upon heating.
• Upon heating react with S, H, C and N.

The common oxidation state of lanthanides is +3, but some of them can have +2 and +4 as well. These elements form complex compounds, hydrides (LnH2), halides (CeF4, TbF4), oxides (Ln2O3, CeO2), hydroxides (Ln(OH)3, Yb(OH)3)), chalcogenides (Ln2S3, Ln2Se3), nitrides (LnN), carbides (EuC2), borides (LnB2, LnB4) and organometallic compounds.

The lanthanides are mostly used as catalyst and in the production of glasses. Cerium is used for lighters production. Some of these elements can be used in nuclear reactors. Furthermore they are used for magnesium alloys, electronic polishers, refining catalysts, lasers, hybrid car components and in optoelectronics. Lanthanum is important for the optical fiber production.

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