Key Questions

1. Why are alkali metals never found in elemental form in nature? [The alkali metals are extremely reactive. More specifically, they react with water, oxygen, the halogens, and in more complex reaction sequences form hydrogen carbonates, carbonates, and other compounds.]
2. How are alkali metals prepared or obtained? [The alkali metals can be prepared by electrolysis of their molten salts:
3. Why are many alkali metal compounds soluble in water? [Solution of a solid is a three-step process. In the first step, ions in the crystal lattice have to be separated. Since alkali metal ions have large ionic radii and only a +1 charge, the coulombic attractive forces in the crystal lattices are weak and more readily separated.]
4. How are the physical and chemical properties of alkali metals related to their electronic structure? [The alkali metal atoms are large and have only one valence electron. Their ions are also large and have a charge of +1. These characteristics lead to relatively weak attractions between the atoms in the alkali metals and the alkali metal ions in their compounds. Alkali metals more readily lose their electrons than other elements, leading to high chemical reactivity with elements and compounds.]
5. How do physical and chemical properties of alkali metals vary in the group? [As the atoms increase in size as one goes down the column in the Periodic Table, the attractive forces decrease and in general so do the numeric values of properties such as melting point, boiling point, hardness. Correspondingly, the decreasing first ionization energy trend from Li to Cs leads to increased chemical reactivity going down the alkali metal family.]
6. Why are reactions involving elemental alkali metals oxidation-reduction reactions? [Reactions involving the chemical elements are all oxidation-reduction reactions. In the case of alkali metals the general reaction is M° -->M⁺ + e^­ .]
7. Why are reactions involving alkali metal compounds rarely oxidation-reduction reactions? [Alkali metal compounds contain large M + ions, which add electrons with difficulty to form the elements.]
8. Why are reactions involving compounds of alkali metals almost all acid-base, precipitation or redox reactions of the anionic constituents? [The large alkali metal +1 ions have weak coulombic electrical force fields and only weakly attract anions. Hence they form few precipitates. The large hydrated alkali metal ions have a very low tendency to function as acids by donating protons (from the H 2 O ligands). Since alkali metal ions (see #7) have little tendency to gain electrons they undergo few oxidation-reduction reactions. Instead the reactions of the compounds involve the anions. For example:

Precipitation Na₂SO₄(aq) + BaCl₂(aq) --> BaSO₄ (s) + 2NaCl(aq)

Acid-base 2NaOH(aq) + H₂SO₄(aq) --> 2H₂O(l) + Na₂ SO₄(aq)Na₂CO₃(aq) + 2HCl (aq) -->CO₂(g) + H₂O(l) + 2NaCl(aq)

Redox 5 Na₂C₂O₄(aq) + 2KMnO₄(aq) + 8H₂SO₄(aq) -->10CO₂(g) + 2MnSO₄(aq) + K₂SO₄(aq) + 5Na₂SO₄(aq) + 8H₂O(l)]

9. What is the crystal structure of alkali metals? [Alkali metals crystallize in body-centered cubic lattices in which only 68% of the unit cell is occupied by cations in the unit cell.]

10. Is there a relationship between physical properties of alkali metals and crystal structures? Explain. [As indicated in Question 5, the attractive forces among atoms in alkali metals decrease going down the column resulting in a decrease in the magnitude of all properties related to coulombic attractive forces.]

11. Why do alkali metals have such low ionization energies? Explain using CoulombÕs law. [Alkali metals have large atomic radii and as a consequence the outer valence electron is only weakly attracted by the core charge of +1. These attractive forces decrease from Li ⁺ to Cs ⁺as the ionic radii increase. Since in the Coulomb's Law equation (see Language of Chemistry), atomic radius is in the denominator, increasing radius leads to decreased attractive force for the valence electron.]

Counterintuitive Examples and Discrepant Events

In the halogen family the largest atom, At, and the molecule At₂ are the least reactive.

The largest atom in the alkali metal family, Cs, is the most reactive.

Pictures in the Mind

(See Transparency Master in the Appendix: Cross Section of Atoms and Ions.)

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