Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Summer 2006
Lecture Notes::Lec 18_23 June	© R. Paselk 2006
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The Representative Elements, cont.

Chemistry of the Alkali Metals, cont.

• Reactions: All of the alkali metals give ionic salts with the exception of a few lithium compounds.
  • Water: all react to give hydrogen gas and base (hence the name alkali metals), e.g.:
    • 2Na_(s) + 2H₂O_(l) 2Na⁺_(aq) + 2OH^-_(aq) + H_2(g)
  • Oxygen: all react with gaseous oxygen, but give different products:
    • Lithium give an oxide: 4Li_(s) + O_2(g) 2Li₂O_(s)
    • Sodium gives a peroxide: 2Na_(s) + O_2(g) Na₂O_2(s)
    • Potassium, Rubidium and Cesium give superoxides: K_(s) + O_2(g) KO_2(s)
  • Lithium is the only element on the Periodic chart which reacts with nitrogen at room temperature (other alkali metals react at higher temps), giving lithium nitride:
    • 6Li_(s) + 2N_2(g) 2Li₃N_(s)
• Compounds: The alkali metals form a wide range of compounds.
  • The salts are generally soluble. Sodium and potassium salts are so soluble they generally cannot be readily analyzed by wet chemical means (they are usually analyzed by flame tests or electrochemical means).
  • The oxides and hydroxides are very basic.
  • Only lithium salts typically crystallize as hydrates.
  • Alkali metals occur along with other metals in a wide range of minerals.

Group II - the Alkaline Earth Metals

Physical Properties

• Valence shell electron configurations: all ns².
• Ionization energies and oxidation states: If you look at the values in the table below one might expect to see +1 as well as +2 as an oxidation state, or, even +3. Why don't these values show up?
  • If we were to look at the atoms we would see that, like Na the atom has a diffuse valence cloud of electrons surrounding a tight core. A dot density image of a beryllium atom is shown below:

  

  • If the Be atom loses a single electron it does not reduce in size all that much, since there is still an electron in the valence shell, as a result the ion cannot pack as tightly in compounds nor approach a negative counter ion as closely as the +2 core.
  • The difference in energy due to being able to bring the ion much closer is greater than the energy to remove the second electron, so +1 ions are not stable. (The only place +1 ions of Group II are observed as "stable" species is in high vacuum situations, where ion concentrations are very low.)
  • A similar argument holds in solutions, where the hydration energy of the +2 ion is much higher (the waters can pack closer), so the energy difference drives the formation of the +2 ion and the +1 ion is not stable.
• Note properties in table, discussed mp, density, ionization energies, reduction potentials and chemistry etc.

Chemistry

• Reactive metals with electronic configurations of: [noble gas]ns²
  • Readily lose outermost electrons to attain a noble gas configurations.
  • Much too reactive to be found free in nature.
  • Reactivity generally increases from Be to Ba.
• Group II elements other than Be are very metallic.
  • Relatively soft metals, but not as soft as Alkali metals.

Beryllium is exceptional, as are other Period 2 elements as a result of small radii and consequently strong bonding.

• Be²⁺ ionic radius is similar to that of Al³⁺, giving them some chemical similarities
  • This is a common phenomena on the Periodic table, with a diagonal relationship occurring between the first member of a group and the diagonal member of the third group.
• All Be compounds have significant covalent bonding.
• Be²⁺ does not exist as such in either ionic crystals or in solution.
• Be is fairly unreactive at room temperature.
• BeH₂ forms a polymer, with two hydrogens bridging each Be in Be-H-Be bonds (tetrahedral).
  • The Be-H-Be bond is known as a three center bond. Only one electron pair is involved in this bond! (Can only accommodate one pair around H.)
• Be forms halogen compounds of the formula BeX₂. As an example;
  • BeCl₂ is linear.
  • Note Be has only a tetrad of electrons around it, not the classic octet!
  • As expected from VSEPR theory BeCl₂ is linear, however, in its crystal structure a chain is formed with each Be in the center of a tetrahedron with Cl's at the corners (see figure in text, p 925)
  • Be in BeCl₂ is very electron deficient, and is thus a good Lewis acid, reacting with Lewis bases such as ammonia: BeCl₂ + NH₃ Cl₂Be-NH₃
• Be is found in mineral beryl (gem quality with trace amounts of chromium to give green color = emerald). Used to harden copper alloys (used in spark-free tools etc.). Toxic as element and in compounds.

Properties of the remaining Alkaline Earths are more similar.

• Magnesium has both covalent and ionic compounds:
  • Grignard Reagents in OChem: R-Mg-Cl, where R- = a carbon aduct such as CH₃CH₂-.
    (e.g. CH₃CH₂Mg-Cl + CO₂ + HOH CH₃CH₂CO₂H + Mg²⁺ + Cl^- + OH^-)
  • Magnesium is found in nature as dolomite, CaMg(CO₃)₂, as well as carbonates and silicates and in sea water.
  • It is an essential element in biology as a cofactor for NTP metabolism.
  • Valuable metal in light-weight alloys.
• Calcium and the remaining Group II elements form ionic compounds with oxidation states of +2.
  • Calcium is found in vast deposits of limestone and marble formed from ancient calcium carbonate plankton shells.
  • Calcium and "hard water."
    • Soap scum: Divalent ions such as calcium can "cross-link" soap molecules, e.g with sodium stearate:
  • 2CH₃(CH₂)₁₅CH₂COO^-Na⁺_(aq) + Ca²⁺_(aq) CH₃(CH₂)₁₅CH₂COO^-Ca^2+-OOCCH₂(CH₂)₁₅CH_3(s)
    • Boiler scale: Ca²⁺_(aq) + 2HCO₃^-_(aq) CaCO_3(s) + CO_2(g) + H₂O_(l)
  • Also found as gypsum (CaSO₄*2H₂O) and apatite (Ca₁₀(OH)₂(PO₄)₆), a major component of bone and tooth enamel.

© R A Paselk

Last modified 23 June 2006