Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Fall 2003
Lecture Notes::Lec 30_10 November	© R. Paselk 2003
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The Chemistry of the Elements

The Representative Elements

Group V, cont.

• Phosphorus
  • Occurs as three common allotropes (at least three others also exist):
    • White phosphorus, P₄, a very reactive, poisonous substance. It is a translucent white solid with P₄ molecules in a cubic structure. It is stored under water to prevent its spontaneous combustion in air with the release of large amounts of heat (DH° = -2940 kJ/mol).
    • Red phosphorus has "tubes" of phosphorus in a layered arrangement. It is much less reactive then white phosphorus, is less poisonous, and can be stored in air.
    • Black phosphorus has a sheet structure and is the most stable form at room temperature.
  • Phosphorus in the phosphate structure (PO₄^3-) form is very important in biological systems, particularly in the form of di- or tri- polyphosphate forms. These bonds are thermodynamically unstable, but kinetically very stable unless a catalyst is present. Hydrolysis of these polyphosphate anhydride bonds releases a great deal of energy and serves as the major immediate form of chemical energy in organisms, driving muscle contraction, ion pumping etc.
• Arsenic
  • Occurs as three allotropes
    • Gray: metallic appearance
    • Yellow: non-metallic, consists of As₄ molecules like white phosphorus molecules
    • Black:unstable
  • Occurs as the yellow pigment orpiment (As₄S₆)
  • Very toxic, but essential as a trace element for blood formation. In the form of arsenate (AsO₄^3-), it can substitute for the very similar phosphate in some enzyme sites, and as a result be incorporated instead of phosphate. However, unlike phosphate the bonds with arsenate are readily hydrolyzed, so the energy captured is immediately lost to the organism - not good!
  • In ancient times arsenic was often used in bronze alloys. It took centuries to realize that working with these alloys was very hard on bronze smiths!
• Antimony
  • Occurs as four allotropes
    • Gray: metallic, but poor conductor.
    • Yellow: non-metallic, consists of Sb₄ molecules like white phosphorus molecules
    • Black
    • "explosive antimony" violently converts to gray antimony when scratched.
• Bismuth
  • Dense metal with dull metallic luster.
  • Bismuth is (like water) unusual in expanding on freezing. Some of its alloys share this property and are used where very tight fits are needed (such as in making type from type metal.)

Group VI

Chemistry

Group VI consists of Oxygen, Sulfur, Selenium, Tellurium, and Polonium. Again we see a tend towards increased metallicity as we go down the group. Oxygen and sulfur are case-study non-metals with no metallic properties of note. Selenium and Tellurium are also non-metallic, behaving much like sulfur (below), however, tellurium has some metallic tendencies. Finally, Polonium is somewhat metallic, considered a metal by some, and forms a basic oxide, but its chemistry is not well known since it has no stable isotopes.

Most commonly the Group VI elements pick up two electrons to give the inert gas valence shell configuration. They tend to form 2- ionic compounds with metals, as we've seen in lab for the sulfides.

Oxides and sulfides are the most common minerals formed by most metals (e.g. galena, PbS, pyrite, FeS, iron oxide, FeO+Fe₂O₃).

They form covalent compounds with non-metals (e.g. H₂O, H₂S, SO₂, SF₄, etc.).

• Note that all but oxygen form compounds with expanded valence shell, such as SF₆.
• Tellurium and Polonium can also form +4 cations, showing the increasing metallicity of the group.
• Oxides of all but polonium and oxygen are acidic.

• Oxygen: We have already studied much of the chemistry of oxygen in a variety of guises (oxides, oxyacids, water, etc.) .
  • Two common allotropes, oxygen (O₂) and ozone (O₃). Both reactive gases, but ozone is an even more powerful oxidizing agent, doing immense amounts of damage in cities, but is protective in the stratosphere as an ultraviolet absorber.
  • -2 (oxide) is the predominant oxidation state, but can also have -1 (peroxide) and -1/2 (superoxide). Positive oxidation states occur only in compounds with fluorine and are of mostly formal interest to us in the oxidation state rules.
  • Oxygen is isolated from liquid air by fractional distillation, having a boiling point (-183 °C) slightly below that of nitrogen.
  • It is the most common element in the Earth's crust, and the third most common element in the Universe (after hydrogen and helium).
  • It comprises about 21% of the Earth's atmosphere by volume.
    • Nearly all free oxygen is a result of biological processes.
    • As a powerful oxidizing agent free oxygen is used by the majority of organisms to provide energy via respiration.
    • As a powerful oxidizing agent oxygen is also very toxic, and organisms have to have a variety of protective systems to survive in its presence.
  • Most mineral ores are either oxides or sulfides.
    • Many oxide ores are a result of biomineralization as a result of metabolic processes of microorganisms.
  • Most human energy is obtained via combustion with oxygen, both metabolically and technologically.

© R A Paselk

Last modified 11 November 2003