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Lithium Group (Group IA) Trends

Summary of Alkali Metal (Group 1) Trends:

1. All metals are malleable and they become softer down to column. Li, cut w/ knife; Rb/Cs consistency of putty

2. Reactivites of metals towards O2 and H2O increases down the column. H2O rxn: Li slow, Na vigorously, K evolves H2, Rb/Cs explosive Orxn (heat) products: Li2O, Na2O2, KO2, RbO2, CsO2.

3. Li does not replace proton on PhC=CH, whereas remaining elements do.

4. Wide range of salts with ionic properties formed:

  • high melting points
  • water soluble that give conducting solutions
  • halide salts are ionic and not hydrolyzed
  • oxides and hydroxides are basic
  • hydrides are ionic, basic, and strong reducing agents

5. The thermal stabilities of the carbonates, nitrates, sulfates, peroxides, and superoxides increase down the column.

6. The reverse order of reactivity (from 5) is observed for the reactions of alkali metals with N2 and carbon, due to high lattice enthalpies of N3- & C22- lattices: Li + N2 Li3N (purple, moisture sensitive) Li2C2 or Na2C2

7. The high charge/size ratio for lithium leads to anomalies

  • LiH more stable than other hydrides</span>
  • Li2CO3 so much less stable than other carbonates in group
  • Li salts less soluble in H2O, more soluble in organic solvents
  • Li/Na form a number of hydrates salts, K some, Rb/Cs none

8. Solubilities of the hydroxides in water increase down the column: LiOH < NaOH < KOH < RbOH < CsOH

9. Alkali metal ions do not readily form complexes with ligans such as NH3 or CN-, but polydentate ligands (chelates)such as crown ethers and cryptands (see text) with oxygen and nitrogen donor atoms (Lewis Bases) do form stable complex ions. The bonding in the complex ions is primarily electrostatic (ionic, very hard) and the relative sizes of the cation and the cavity are important. For example, for complex ions with the 18-crown-6 ether ligand the order of stability is: Li+ < Na+< K+ > Rb+ > Cs+ different ring sizes prefer different ions: Li+, 12-crown-4 & 15-crown-5; Rb& Cs+, 24-crown-8

10. Lithium alkyls are widely used as reagents in organic chemistry because they are readily synthesized, they are soluble in organic solvents, and (for Chem 261/262) they provide a slightly more reactive source of carbanions than Grignard reagents (Mg alkyls). These properties arise because alkyl lithium compounds form oligomers (clusters) in the form of rings and polyhedra which have the hydrocarbon residues dominating the surface. In contrast, sodium and potassium alkyls are more ionic, generally adopt infinate 3-D structures in the solid state, are not soluble in hydrocarbons, and are extremely air/moisture sensitive.