Lithium Characteristics
Lithium (atomic number 3, atomic weight 6.941) is a silvery-white metal, slightly harder than sodium but softer than lead. It is extremely light. Lithium appears in the periodic table as the first element in Group I, the alkali metals group. Like the other metals in the group - sodium, potassium, rubidium and cesium - it is so chemically active that, in nature, it never occurs as a pure element, but is always bound in stable minerals or salts.
In some of its compounds, however, lithium shows a great resemblance to Group II, or alkaline earth metals. For example, the water solubility of lithium hydroxide is substantially lower than that of other alkali hydroxides. In general, lithium's physical and chemical properties stem from its atomic structure. A single atom of lithium consists of a nucleus (three protons and either three or four neutrons) with three electrons orbiting in two shells. The inner shell (the helium shell) contains two electrons and is chemically inert. The outer shell contains only one electron.
Lithium, more than any other alkali metal, tends to eject this electron from its outermost shell. The resulting lithium-ion carries a positive charge (+1). In solid metal, individual lithium atoms are arranged geometrically in a cubic lattice and can transfer a negative charge from place to place. This electron movement makes lithium metal an excellent electrical conductor.
Lithium derives its excellence from the following important characteristics:
- Low density
- Low Melting point
- Soft and easy to form
- Low dynamic viscosity
- Very high ionization energy
- Very high electrode potential
- Very good electrical conductivity
- Low resistivity
Very reactive, though considerably less so than other alkali metals. The presence of sodium as an impurity, even in amounts of 0.5 – 1 %, increases its reactivity, e.g., for the formation of lithium alkyls from lithium metal and organic halides.
A freshly cut surface of lithium metal has a silvery luster. At room temperature in dry air with a relative humidity of less than 1 %, the surface remains shiny for several days, although a very thin passive surface layer is formed that is hardly visible to the naked eye and consists mainly of lithium carbonate and oxygen-containing compounds. Lithium metal can therefore be processed in dry air. However, in moist air a dull gray coating, consisting mainly of lithium nitride, lithium oxide, and lithium hydroxide, forms within a few seconds. If lithium ingots are allowed to remain in contact with air for some weeks, the reaction with atmospheric nitrogen extends into the interior of the metal with the formation of reddish brown lithium nitride and can lead to ignition. Even at room temperature dry nitrogen reacts slowly with lithium metal.
Protective gases for lithium metal include the noble gases, or pure sulfur hexafluoride up to 225 °C. Mineral oil is also suitable as a protective medium.
Lithium reacts with water with formation of hydrogen, which ignites under normal conditions only if the metal is finely divided. Molten lithium reacts explosively with water.
Lithium reacts with hydrogen to form lithium hydride. This reaction is carried out on an industrial scale at 600 – 1000 °C. Lithium reacts with gaseous ammonia at elevated temperature to form lithium amide. The vigorous reaction with halogens produces incandescence. Organic compounds containing active hydrogen or halogen usually react with lithium to form the corresponding organolithium derivative.