The Inorganic Chemistry of Lithium

Lithium is a member of group 1 (1A) alkali metals, has an atomic number of 3, an atomic weight of 6.941 and contains a single valence electron. In its pure form it is a soft silvery white metal, turning grey on exposure to air. Because of its reactivity in the metallic state, it is only found in nature as an ore or at low concentrations as a soluble salt, e.g. it is present in river waters at ^0.002 mg/l. But it has also been found at higher levels in naturally occurring springs which have previously been associated with health improvements (e.g. Lithia Springs Mineral Water, Georgia, USA).

The most important commercial ore containing lithium is spodumene, LiAl(SiO3)2. The extraction of lithium from this ore occurs by converting it to the soluble chloride salt. This process starts by heating spodumene to around 1100 °C. This is then mixed with sulphuric acid and water to form lithium sulphate (Li2SO4). This is washed with sodium carbonate (Na2CO3) to form a precipitate of the lithium carbonate (Li2CO3) (Equation 1.1). Reaction of lithium carbonate with hydrochloric acid gives rise to lithium chloride (Equation 1.2).

Li2SO4 + Na2CO3 Na2SO4 + Li2CO3 (1.1)

As a salt, lithium has the smallest ionic radius, the largest field density at its surface and is the least reactive of the alkali metals. The very small diameter of the lithium ion in relation to water means that molecular packing gives it an anomalously large hydration shell in proportion to other group 1 elements. Thus in experiments examining the effect of lithium salts on osmolarity, the corresponding sodium salt is often used as a control, as this provides the closest approximation to lithium's hydration shell size.

Although the hydration shell of lithium is similar in size to sodium, its ionic radius is much closer to that of the magnesium ion (Table 1.1). This similarity allows lithium ions to compete with magnesium ions for binding sites in pro-teins.3 5 As magnesium ions interact with hundreds of proteins, this suggests that lithium may inhibit numerous enzymes. However, the relative specificity of the lithium effect suggests that only proteins with very low affinities for magnesium are targeted by therapeutic concentrations of the drug. The total magnesium concentration of the cell is between 10 and 15 mM, but the free concentration of magnesium is between 0.5 and 1.0mM. This is the same concentration of lithium found in plasma from patients undergoing lithium therapy.6

Table 1.1 The chemical symbol, atomic charge and atomic number of lithium and its neighbouring elements on the periodic and their ionic radii table in different coordination states. Units are given in pico moles

Lithium

Beryllium

Boron

Sodium

Magnesium

Symbol

Li

Be

B

Na

Mg

Charge

I

II

III

I

II

Atomic no.

3

4

5

11

12

4 coordinate

73

41

25

113

71

6 coordinate

90

59

41

116

86

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