Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Summer 2006
Lecture Notes::Lec 19_26 June	© R. Paselk 2006
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The Representative Elements, cont.

Group 2 Chemistry, cont.

• Strontium and Barium: Like Ca, compounds are ionic with oxidation state of 2+
  • Strontium has similar characteristics to calcium, however it forms more insoluble precipitates with phosphate. This can be very important in that Sr replaces Ca, for example in bone. This partially explains the particular danger of the radioisotope ⁹⁰Sr, which is produced in atomic bomb tests.
  • Ba, which is quite toxic as an ion, forms a salt, BaSO₄, which is harmless because it is so incredibly insoluble.

Group III

Chemistry

Group III, Boron, Aluminum, Galium, Indium, and Thallium, introduces a couple of characteristics that will continue in varying degrees with the other p-block elements.

1. The higher oxidation state decreases in stability as we go down the group. Thus for Group III we see the +3 oxidation state is important for all Group III elements, and the only oxidation state seen for example for Al, +1 is also an important oxidation state for Tl.
2. The metallic character of the elements for identical oxidation states increases as we go down the group.
   • Thus B is a semi-metal, and its oxide, boric acid [B(OH)₃], is a weak acid, rather than a base as we saw for the metals in Groups I & II. Its a bit different than what we've seen before in that it acts as a Lewis acid, reacting with water

   B(OH)_3(aq) + H₂O_(l) B(OH)₄^-_(aq) + H⁺_(aq)

   • Al and Ga are both amphoteric - that is they dissolve in both strong aqueous acids and bases:

   2Al_(s) + 6H⁺_(aq) 2Al³⁺_(aq) + 3H_2(g)

   2Al_(s) + 6H₂O_(l) + 2OH^-_(aq) 2Al(OH)₄^-_(aq) + 3H_2(g)

   • In and Tl do not react with strong bases, but react with strong aqueous acids. Their hydroxides are basic as expected for a metal.
   • Galium, Indium and Thallium are not as metallic as one might expect due to the filling of the d-shells of the transition elements. Remember, these are internal shells, so the nucleus is adding charge, causing the atom to shrink and thus bring the outer electrons in closer. As a result the valence electrons are not as easily lost - less reactive and less metallic.

Properties of Group III

Property		B	Al	Ga	In	Tl
Outer electron configuration		2s22p1	3s23p1	4s23d104p1	5s24d105p1	6s24f145d106p1
Melting point (°C)		2300	660	29.7	156	304
Density (g/cm3)		2.37	2.70	5.90	7.3	11.9
Ionization energies - 1st & sum of 1-3 (kJ/mol) M(s) M3+(aq) + 3 e-		800.6 6886	577.6 5137	578.8 5520	558 5063	589.3 5415
Standard Reduction Potentials (V, 25°C) M2+(aq) + 2 e- M(s)		-0.87	-1.66	-0.53	-0.34	-0.72
Electronegativity		2.0	1.6	1.8	1.8	1.6

• Boron:
  • As noted above B is a semi-metal, and shares properties and chemistry with the diagonally related semi-metal, silicon.
  • Most of boron's compounds are covalent.
  • The covalent hydrides are called boranes and are particularly interesting in their bizarreness:
    • BH₃ is predicted to have a sextet of electrons around boron giving a trigonal planar molecule.
    • However, BH₃ is unstable, reacting with itself to form diborane, B₂H₆, where the two borons are connected via hydrogen bridges with three-center bonding as we saw with beryllium hydride.
    • Higher boranes also exist. In some of them we see not only three-center bonding, we also see five-center bonding.
    • Boranes are highly unstable due to their extreme electron deficiency. Their highly exothermic reaction with oxygen lead to their consideration as rocket fuels by the space program.
  • Boron nitrides is similar to diamond in hardness, strength and sparkle, and it is produced in large quantities as an abrasive and coating.

  All of the Group III metals (Al, Ga, In, & Tl) form both covalent and ionic bonds.

  • All four metals form halogen compounds with the formula MX₃
    • The fluorides, MF₃ are ionic.
    • The chlorides, bromides and iodides are low melting point compounds with dimeric vapor-phase molecules, however the situation is a bit more complex than this.
      • For example, aluminum chloride crystallizes from aqueous solution as an ionic salt containing hydrated Al³⁺: [Al(H₂O)₆]Cl₃, commonly written as AlCl₃*6H₂O.
      • On the other hand, anhydrous aluminum chloride forms a 3-D covalent network with chlorides bridging the aluminums.
      • In the vapor phase, aluminum chloride exists as the dimeric Al₂Cl₆, with the aluminums bridged by chlorides (Cl₂AlCl₂AlCl₂).
  • Well defined M³⁺ ions exist as hydrated forms in acid solution, e.g.: Al(H₂O)₆³⁺.
  • Aluminum is the most abundant metal and the third most abundant element in the Earth's crust.
    • Very metallic in elemental form.
    • Bonds to nonmetals have significant covalent character.
      • This results in amphoterism noted above.
      • Aluminum forms a hard, tough coating of aluminum oxide (Al₂O₃) which protects the metal from corrosion. A thicker coating may be applied electrolytically to give an anodized surface (dyes added to this coating can give it colors).
      • We already discussed aluminum production by the Hall process in electrochemistry.
      • Aluminum reacts with hydroxide to form aluminum hydroxide, a gelatinous solid: Al³⁺ + 3OH^- Al(OH)_3(s)
      • Alumina (corundum), Al₂O₃, forms a crystalline solid with oxygen in a hexagonal close packed array, and aluminum in two thirds of the octahedral holes. However, the Al-O bonds have significant covalent character, making alumina extremely hard (9 on Mohs scale; e.g. aluminum oxide sandpaper).
        • Alumina (corundum) base of various precious gemstones, e.g. sapphires (traces of Fe²⁺ and Ti⁴⁺ gives blue, Fe³⁺ gives some yellows, pure gives "white" or colorless, other metals give other colors), ruby (traces of Cr³⁺), padparadscha (orange/pink, traces of Fe^(3+?) and Cr³⁺).
        • Alumina is very stable, as demonstrated in the Thermite reaction:

      2Al + Fe₂O₃ Al₂O₃ + 2Fe

  • Gallium, Indium and Thallium are all rare, found primarily in zinc, lead and cadmium, ores.
    • Gallium has a low melting point but a very high boiling point making it the element with the widest liquid range known. It is thus used in some thermometers, and it is used in semiconductors, such as galium arsenide lasers.
    • Thallium is the only element in Group III and the first element we've looked at to exhibit the so called inert pair effect. This refers to the fact that the s-electrons would appear not to participate as readily in ionization, giving a +1 ion as well as the +3 ion (as you might expect its more complex). Thallium(I) is extremely toxic, mimicing potassium (acts as a neurotoxin) and being recycled in the intestinal tract so that it is difficult to get rid of.

Group IV

Chemistry

Group IV shows a very obvious transition from a non-metal to increasingly metallic elements going down the group, ending in true metals.

• Carbon is a classic example of a non-metal.
• Silicon and Germanium are semi-metals.
• Tin and Lead are metals.

Group IV give perhaps the most obvious example of the difference in properties between elements of Period 2 and higher Periods, since carbon, as a very distinct non-metal, behaves much differently than any of the other Group IV members.

The elements from silicon to lead show a nice transition of properties towards increasingly metallic.

The group shows an obvious "inert pair effect" with silicon and germanium exhibiting the +4 oxidation state, while Tin and lead exhibit both +2 and +4 oxidation states.

• Carbon
  • Very abundant in both elemental and combined form in Earth's crust. Three Allotropes (different physical forms of the same element)
    • Diamond: colorless (when pure) cubic or octahedral crystals (often have curved edges and faces due to lattice defects). Diamond is a non-conductor of electricity since all valence electrons involved in covalent bonds
    • Graphite is electrically conductive, and has a semi metallic luster, so has a bit of metal character!
    • Fullerenes
  • +4 and -4 oxidation states
  • One of only two elements with highly stable and strong single, double, and triple bonds.
  • Important inorganic compounds of C include CO, CO₂, CN, carbonates, cyanides, and carbides
    • Various carbides are important as abrasives and cutting tools (silicon carbide, tungsten carbide, etc.)
  • Infinite number of carbon based compounds due to very strong and stable C-C bonds.

© R A Paselk

Last modified 26 June 2006