Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Fall 2003
Lecture Notes::Lec 28_5 November	© R. Paselk 2003
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The Chemistry of the Elements

The Representative Elements

Group IV, cont.

• Silicon. cont.
  • Extended silicate ions are called metasilicates. Note that in each case the silicon is again bonded tetrahedrally to four oxygens.
    • The simplest metasilicate is the "infinite" linear chain, SiO₃^2-, as noted last time.
    • The next silicate has two chains linked via bridging oxygens to give a band structure (double chain). In this case the empirical formula is the Si₄O₁₁^6- ion.
      • This structure occurs in a variety of minerals known as amphiboles, including the various asbestos minerals.
    • Silicate can also form sheets with the empirical formula Si₂O₅^2-. In this case each silicon atom is crosslinked to three others via oxygen bridges, with a single free oxygen.
      • When these sheets are cross bridged to each other via cations they form a variety of sheet structured minerals such as talc [Mg₃(Si₂O₅)₂(OH)₂], micas, and clays. These minerals are critical as a reservoir holding cations in soils essential to plant growth.
    • Silica can also form cyclic ions with the oxygens bridging the silicons, leaving two free oxygens per silicon (thus each silicon is in the center of a tetrahedron, and has two negative charges, one each on the non-bridging oxygens), such as
      • cyclic-trimetasilicate (Si₃O₉^6-, a six-membered ring structure), which occurs in benitoite (BaTiSi₃O₉).
      • cyclic-hexametasilicate (Si₆O₁₈^12-, a twelve-membered ring structure), which occurs in beryl (Be₃Al₂Si₆O₁₈).
    • Quartz has the empirical formula of SiO₂.
      • Quartz glass is made up of 3-D clusters of SiO₂.
      • Other glasses consist of random arrays of these clusters with other components to ease workability etc. such as
        • soda Na₂O, CaO,
    • Finally, silica chains which have been, for example, methylated on the free oxygens form a valuable series of synthetic compounds, the silicones (e.g. silicone rubber, silicon wax, silicon oils) which are more resistant to breakdown than carbon based oils and waxes.
• Germanium Like silicon it crystallizes as the pure element in a diamond lattice, forming a brittle solid.. It is an important semi-conductor.
  • The chemistry of germanium is much like silicon's, but it can also form octahedral compounds with six oxygens around germanium due to its larger size.
• Tin Fairly rare metal occurring as the ore cassiterite, SnO₂, from which the metal is obtained by reduction with carbon: SnO_2(s) + 2C_(s) Æ Sn_(l) + 2CO_(g). It is used in various alloys (pewter, bronze and solders) and protective plating of "tin cans" etc.
  • Three allotropes:
    • a-tin (gray tin): non-metallic, stable below 13°C, atoms bonded in diamond lattice. "Tin disease."
    • b-tin (white tin): the common, metallic form, stable from 13°C - 161°C.
    • g-tin (rhombic tin): atoms are bonded in an orthorhombic lattice, brittle, stable above 161°C to the melting point of 232°C.
  • There are two oxidation states, +4 (stannic) and +2 (stannous). The second common oxidation state of +2 demonstrate the inert pair effect.
    • Note (and know) chemistry we see in lab.
• Lead Obtained from the mineral galena (PbS, often occurs in shiny, silvery, cubic crystals). may be obtained from ore by roasting in oxygen to give lead(II) oxide (PbO) and lead(II) sulfate which can then be mixed with additional PbS without air to give lead metal. It is important in alloys and solders, and particularly in lead acid batteries. It used to be used in vast quantities in leaded gasoline. Important from ancient times
  • Like tin, lead exhibits two oxidation states, +4 (plumbic) and +2 (plumbous). The second common oxidation state of +2 demonstrate the inert pair effect.
  • Like most heavy metals lead is toxic (Romans).

Group V

Chemistry

Group V consists of Nitrogen, Phosphorus, Arsenic, Antimony and Bismuth. Again we see a transition from non-metals (N & P) through semimetals (As & Sb) to a metal (Bi).

• Group V shows a good illustration of the trend from acidic to basic oxides going down the group:
  • Nitrogen, phosphorus and arsenic form acidic oxides as expected for non-metals: e.g. HNO₃, H₃PO₄, H₃AsO₄.
    • Arsenic(III) oxide gives arsenite (from arsenous acid): As₄O_6(s) + 12OH^- Æ 4AsO₃^3- + 6H₂O
    • Arsenic(V) oxide gives arsenate (from arsenic acid): As₄O_10(s) + 12OH^- Æ 4AsO₄^3- + 6H₂O
  • Antimony forms amphoteric oxides (though a semimetal, its more metallic than arsenic).
    • Basic: Sb₂O_3(s) + 6H⁺ + 6Cl^- Æ 2SbCl_3(s) + 3H₂O
    • Acidic: Sb₂O_3(s) + 6OH^- Æ 2SbO₃^3- + 3H₂O
  • Bismuth forms a basic oxide, Bi(III) oxide, Bi₂O₃, as expected for a metal.
    • Bi₂O_3(s) + 6H⁺ + 6Cl^- Æ 2BiCl_3(s) + 3H₂O
• Group V elements commonly form three different molecular structures:

Molecule Hybridization of M Shape MX3 (e.g. PCl3) sp3 Trigonal pyramidal MX5 (e.g. PCl5) dsp3 Trigonal bipyramidal MX6 (e.g. PCl6-) d2sp3 Octahedral

Properties of Group V

Property		N	P	As	Sb	Bi
Outer electron configuration		2s2p3	3s2p3	4s23d104p3	5s24d105p3	6s24f145d106p3
Melting point (°C)		-210 (bp = -196)	44.1 (white)	814	613	271
Density (g/cm3)		0.88 (liq)	1.82 (white)	5.78	6.70	9.8
Electronegativity		3.0	2.1	2.0	1.9	1.8

• Nitrogen
  • Major constituent of the atmosphere (78% by volume)
  • nitrogen is the second ranked industrial chemical in the US
    • produced by distillation from liquid air.
    • Nitrogen can have a wide variety of oxidation states:
  • Most inorganic nitrogen compounds are soluble, so not commonly found in mineral deposits (exception is in very dry climate locales - current or fossil)

© R A Paselk

Last modified 6 November 2003