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CP Sci 9- 1st Sem.

Gen Chem - 1st Sem.

The Shapes of Small Molecules:


    1.    Define VESPR theory and explain its relationship to the shape of molecules.

    2.    Name and describe 5 basic shapes of molecules.

    3.    Define the term hybrid orbital and explain how it differs from atomic orbital.

    4.    Explain two important trends in bond length.

Key Terms:

    ball-&-stick model    valence-shell electron pair repulsion theory    bond angle    hybrid orbital

Notes: (8-1)

Molecular Geometry:

As we learned in the previous chapter the structural formula of a molecule can give a clear picture of the arrangement of atoms in a molecule.  Its limitation is that it gives only a one dimensional picture of the molecule.  This is why many chemists prefer ball-and-stick models. Ball-and-stick models let us see the molecules in three dimensions which aids in our understanding of the characteristics of the molecules.  There are 5 basic shapes that can be easily displayed by this model system.  

bulletShapes are determined by the VSEPR (valence shell electron pair repulsion theory) which states that in a small molecule, valence electrons pairs will configure themselves as far apart from each other as possible.
bulletThis configuration represents the lowest amount of energy possible required to maintain the shape of the molecule.

Consider the following examples:

Ammonia (NH3) - pyramidal   



(members.tripod.com/~EppE/ jpgs/ammonia.jpg)

Carbon dioxide (CO2) - linear                                       

[Carbon Dioxide (CO2) Molecule - Click for a 3D viewable model of this molecule. Note: You must have a VRML viewer/plug-in to see it! - It opens in a new window.]

(www.bacharach-europe.com/.../ gasdt_carbon-dioxide-co2.jpg)

Boron triflouride (BF3) - trigonal planar                    



(www.wbateman.demon.co.uk/summaries/ sum3/trigplan.jpg)



Methane (CH4) - tetrahedral



(members.tripod.com/~EppE/ jpgs/methane.jpg)


Water (H2O) - bent   




Bonding of atoms can be further explained by the concept of hybrid orbitals.  A hybrid orbital describes the interaction of the valence electrons an atom as they approach another atom.  Consider the following chart:


(chemed.chem.purdue.edu/.../bp/ ch8/graphics/fig8_28.gif)

You will notice as electrons become available the orbitals assume the shape that best fits with the VESPR model. Here are some examples for you.

bulletsp - NaCl
bulletsp2 - BF3
bulletsp3 - CH4, H2O

Bond Length:

Trends in bond length give us an idea of the reactivity of certain compounds.  Here are some simple rules to follow when determining bond length.

bulletbonds get shorter as you move down the periodic table
bulletmultiple bonds are shorter than single bonds
bulletshorter bonds are stronger




Last modified: September 05, 2004