Content in a Nutshell
From saline solutions to street lamps, from baking powders to bleaches, from fertilizers to fireworks, alkali metal compounds find everyday uses in our lives. Elements necessary for life, yet ones that react violently with water: do these two sound incompatible? In their aqueous ionic forms, sodium and potassium ions (Na+ and K+) are essential ingredients for animal and plant life, yet the elemental atomic forms of these alkali metals react vigorously with water and other compounds.
The alkali metals (Li, Na, K, Rb, Cs, Fr) form a vertical family of elements that begin each horizontal row of the Periodic Table. Francium, element 87, is a rare radioactive decay product of the radioactive element, actinium. Even the most stable isotope of franciumdecays sorapidlythatits chemicalpropertiesarenot wellknown.Theother alkali metals are silvery metallic solids, as soft as cold butter. Lithium, sodium, and potassium are less dense than water; hence they float on it (but don't try it!). Lithium's density (0.53 g/cm3) is just over half that of water. All have low melting points. Cesium melts at 29 °C, a bit above room temperature; Na melts at 98 °C, just below the boiling point of water. The alkali metals are so highly reactive that they never occur free (in their elemental form) in nature, but always in combination with other elements. Since they react rapidly with oxygen in air and violently with water, they must be stored in unreactive oil or kerosene.
All alkali metals react vigorously with halogens to produce alkali halides. Large quantities of alkali metal chlorides are found in the oceans, inland seas, and salt deposits. Fifty million billion (5 x 1016) tons of salt (NaCl) are dissolved in earth's oceans. The sodium ion, Na+, is the principal positive ion in fluids surrounding cells in our bodies, where it is needed for water retention and muscle action. This ion (in the form of saline solution) is often given intravenously to hospital patients. Potassium ions, K+, are also essential to life, both plant and animal. Compounds of this ion, such as KCl, K2SO4, and KNO3, are used extensively as fertilizers (see Industrial Inorganic Chemistry module). Insolublelithiumcarbonate,Li2CO3, isused to treat manic depressives, although its mode of action is not well understood.
Each alkali metal atom has one more electron than the chemically stable noble gas atom just preceding it in the Periodic Table. Each atom has a large relative size (radius), coupled with filled inner energy levels of electrons. Each atom can therefore readily lose this one electron, forming stable +1ions with noble gas electron configurations. Thus they have low ionization energies. Cesium's ionization energy is so low that visible light can ionize it, permitting its use in photoelectric cells, where light energy is converted directly into electricity.
The chemistry of alkali metals provides a fascinating entry into the field of descriptive chemistry and a perfect introduction to the concept of periodicity of the elements (see Periodicity module).
The similarities in physical and chemical properties of alkali metals reinforce the concept of families of elements and thus serve as an excellent introduction to the Periodic Table (Periodicity module.) Due to their strong tendency to form compounds in which they exhibit only the +1 oxidation state, their chemistry is simple and predictable, yet frequently exciting. The topic could thus serve as an introduction to predicting the products of chemical reactions (Simple Chemical Reactions module). As a consequence of their high solubility in water and their relative abundance in nature, the compounds of sodium and potassium find extensive use. Examples familiar to students include table salt (NaCl), baking soda (NaHCO3), lye (NaOH), potash (K2CO3), soap (C17H35COONa), and detergents (e.g., C12H25OSO3 Na+). All these reasons suggest introducing the topic of alkali metals early. The extraction of the alkali metals from their naturally occurring compounds provides a simple introduction to the concept of oxidation-reduction reactions (Oxidation-Reduction module), while much of the chemistry of the anions in the alkali metal compounds focuses on acid-base (Acid-Base module), precipitation (Solubility and Precipitation module), and redox reactions. Alkali metals can thus be related to several major areas of the typical high school chemistry curriculum.
1. The alkali metal family, consisting of Li, Na, K, Rb, Cs, and Fr, is a highly reactive family of elements. The elements in this family are never found free in nature (as Mo atoms), existing instead in chemical combination with anions as ionic compounds (containing M+ ions).
2. The alkali metals are prepared by electrolytic (electrolysis) or chemical reduction [M+ + e --> Mo].
3. Alkali metal cations have a low charge density and relatively weak attractions for negative ions (anions). As a consequence, most salts of alkali metal ions are very water-soluble, and their hydroxides are very soluble strong bases (alkalis).
4. The attractive forces among atoms in alkali metal crystals are relatively weak. As a result each metal has low density, melting point, heat of fusion, electronegativity, ionization energy, and electron affinity; they are also soft, malleable, and ductile.
5. The chemical and physical properties of these elements are similar (as are those of their ions), and most of their properties change in a regular periodic manner as one goes down the family in the Periodic Table.
6. All reactions of the alkali metal elements (as well as those of almost all elements) are oxidation-reduction reactions.
7. The vast majority of reactions of alkali metal compounds are acid-base reactions involving the anions associated with the alkali metals. In oxidation-reduction and precipitation reactions involving alkali metal compounds, it is almost always the anion originally associated with the alkali metal that is involved in the observed net reaction; e.g.,
8. All alkali metals form body-centered cubic crystals.
9. All alkali metals react vigorously with water, producing the alkali metal hydroxide and hydrogen gas.
10. Alkali metals relatively large atomic size (radii) and single electron far from the nucleus, account for their low electron densities and hence low ionization energies. This observation can be explained using Coulomb's Law , where the force of attration (F) is directly proportional to charge (q) and inversely proportional to atomic radius (r).
1. Chemical symbolism and nomenclature, formulas, atomic and molar masses
2. Equation writing, types of reactions, predicting reaction products
4. Solubility equilibrium
6. Chemical periodicity
7. Atomic structure, electronic structure, Lewis structures
8. Molecular structure, ionic compounds
9. Crystal structure (ionic, metallic)
10. Acid-base reactions
11. Precipitation reactions
12. Oxidation-reduction reactions, electrolysis
1. Manipulative skills:
a. The ability to write chemical formulae, predict reaction products, and balance chemical equations.
b. An understanding of basic techniques involved in qualitative analysis of metal ions.
c. An awareness of special precautions involved in lecture demonstrations and laboratory activities involving free alkali metals and their corrosive compounds.
2. Mathematical and conceptual skills:
a. Mastery of stoichiometric skills, unit analysis, solution concentrations.
b. Mastery of arithmetic skills, including simple algebra, use of exponents, and roots.
c. Working knowledge of Coulomb's law and parameters involved in electrical attractions.
d. Ability to write/draw electron configurations.
e. Ability to draw Lewis-dot structures.
f. Ability to predict bond and molecular polarity on the basis of ionization energy (or electronegativity) and molecular geometry.
g. Ability to predict trends in properties of alkali metals (chemical periodicity) on the basis of atomic structure.
After completing their study of alkali metals, students should be able to:
1. explain why alkali metals are never found in pure metallic state in nature.
2. describe the primary ways in which alkali metals are prepared or obtained.
3. explain why most alkali metal compounds are water-soluble
4. discuss physical properties of alkali metals in terms of atomic structure.
5. describe trends in physical and chemical properties as related to alkali metals.
6. explain why many reactions involving elemental alkali metals are redox reactions, but reactions of compounds of these elements rarely involve changes in oxidation state of alkali metal ions.
7. explain why reactions involving compounds of alkali metals are almost all acid-base reactions of anionic constituents.
8. describe crystal structure of alkali metals.
9. explain physical properties of the alkali metals in terms of their crystal structures.
10. explain the relatively low ionization energies of alkali metals as they relate to Coulomb's Law.
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