06.Periodicity of Behavior;
       Sodium Through Argon		   Previous PageNext Page
       Questions

PROBLEMS

1. The formula for an isolated silicate tetrahedron as it exists in minerals such as olivine is SiO44-. Draw a Lewis electron-dot model of this ion, and indicate why each oxygen atom has a negative charge.

2. From atomic radii given in this chapter, calculate the Si-O bond distance assuming (a) a covalent bond between Si and O atoms, and (b) an ionic bond between Si4+ and O2- ions. How different are these two Si-O distances? (Bond lengths can be measured by x-ray crystallography or spectroscopic methods, but it often is harder to decide how much of an observed bond length "belongs" to each of the atoms it connects. This requires looking at bonds between a given atom and many other different kinds of atoms, and arriving at a self-consistent set of bond radii that will reproduce all the observed bond lengths reasonably well.)

3. Zircon is a semiprecious stone, which sometimes is used to imitate diamond. It is made up of isolated silicate tetrahedra like olivine is, but with the charge balanced by ions of zirconium (a transition metal that we will see more of later) instead of iron and magnesium.

 

The empirical formula of zircon is Zr(SiO4). What is the oxidation number of zirconium in zircon?

4. The mineral melilite is built from silicate units that are two tetrahedra connected together. If one silicate tetrahedron is SiO44-, why is the melilite unit Si2O76- rather than Si2O88-? One of the following is an acceptable formula for one type of melilite:

NaCaAl(Si2O7) or MgCaAl(Si2O7)

Which one of the above is impossible? Why?

5. One of the two minerals that commonly are classified as the semiprecious jade is jadeite, a pyroxene made from infinitely long chains of silicate tetrahedra. Show that the empirical formula for such chains is SinO3n2n-, in which n is the number of tetrahedra in the chain, or more simply, the number of SiO32- units. The empirical formula for jadeite is NaAlSi2O6. Why is this written with Si2O6 instead Of SiO3? (Pure jadeite with this composition is a milky white. The color in green jadeite comes from traces of chromium, as in emerald.)
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