Molecular Architecture Extra Review Questions
1. Why do period 2 elements never form more than four covalent bonds? Why are period 3 elements able to exceed an octet when they form bonds?
2. Energetically speaking, which is the more stable species, a hydrogen atom or a hydrogen molecule? Explain.
3.  Draw electron-dot formulas for HONO (nitrous acid), H2CO (formaldehyde), HOClO (chlorous acid), and HOCl (hypochlorous acid).
4. Using the octet rules draw probable structures for a) anhydrous borax (sodium tetraborate), Na2B4O7, b) anhydrous colemanite, Ca2B6O11.
5.  What would be the skeletal structures for (a) SiCl4, (b) PF3, (c) PH3, and (d) SCl3?
6. What holds the two nuclei together in a covalent bond?
7. What is the minimum number of electrons that would be expected to be in the valance shell of As in AsCl3?
8. What is the Lewis symbol for arsenic, As?
9. Which of the following molecules would be nonpolar:   SbH3, IF3, AsF5, SbCl3 or OF2?
10. For each of the following molecules:
a) draw the electron dot diagram.
b) determine the % ionic character of one bond.
c) state whether the bond should be called ionic, covalent or polar covalent.

ICl      SiF4

11. How do we indicate a bond dipole when we draw the structure of a molecule?
12. Which of the following molecules would be expected to be polar?  (a) SF6   , (b) SO2,  (c) BrCl,  (d) AsH3,  (e) CF2Cl2
13. The boiling point of dimethyl sulphide is 38oC, and that of its isomer ethanethiol is 35oC.
CH3-S-CH3               CH3CH2-SH
dimethyl sulphide        ethanethiol
b.p. 38oC                   b.p. 35oC

a) By considering these structural formulas and relative boiling points, what do you conclude about the importance of hydrogen bonding in compounds containing the thiol (-SH) group?

b) By considering the relative molecular weights and boiling points of ethyl alcohol and ethanethiol, what do you conclude about the relative importance in compounds containing O-H groups? 
14. What molecular shapes are expected when the central atom has in its valence shell 
(a) three bonding electron pairs and one lone pair,
(b) four bonding electron pairs and two lone pairs, and 
(c) two bonding electron pairs and one lone pair?
15. Xenon is one of the noble gases, and is generally quite unreactive. In fact, it was long believed that all of the noble gases were totally unable to form compounds. It came as quite a surprise, therefore, when it was discovered that some compound could be made. One of these is xenon difluoride, XeF2. What would we expect the geometry of XeF2 to be?
16. What are the approximate bond angles in each:   a) NO3-, b) SiF4, c) PH3, d) CO32-,  e) SO32-, f) SO3
g) Co(NH3)62+?

          Answers