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METHANE |
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METHANE |
Methane is one of the simplest organic molecules; a great example of an sp3 hybridized central atom (carbon) that forms its bonds with four hydrogens using four sp3 hybrid orbitals. There are several ways to look at methane. Different representations of the molecule emphasize different aspects of its structure. |
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| spin | transparency |
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Let us start with the framed picture on the left.
The "stick" model (1, upper left) is great for highlighting the bond
distances (1.10 Å), bond angles
(109.5o) and the overall disposition of nuclei in 3-D. In fact,
if models are prepared "to scale," one can measure the intramolecular
distances with a ruler! The color coding (hydrogen white, carbon black)
helps to visualize the extent to which individual atoms (i.e. their electrons)
contribute to the molecule. An alternative stick-and-ball model (2, upper
right) does the same, emphasizing that the mass of the molecule is, in fact,
accumulated in the nuclei. Both these models show the tetrahedral nature
of carbon quite well: the hydrogens serve as corners of a tetrahedron (a
trigonal pyramid with all edges of equal length). You may appreciate the
shape of methane better by playing
with the 3D model on the right (or with models build from your model kit). You
may press a button to view the specific model, or you may
accomplish the same (and more!) with the mouse-activated menus over the
spinning molecule. Do not be afraid to
explore!
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The deficiency of the "stick" or "ball-and-stick" models is in their inability to show that molecules occupy substantial amount of space. This shortcoming is absent in the "space-filling" model (3, upper center). Here the extent of the electron cloud is specified and color coded (white hydrogens, black carbon). In such representations it is assumed that atoms occupy the same amount of space in different compounds (not a bad assumption). The spheres of hydrogens and carbon are brought to the bonding distance and allowed to interpenetrate. The resulting "blob" shows well the effective size of the molecule; the surfaces shown are like "borders" for molecules. If two molecules touch, that's how close they can get to each other. We would say that they remain in van der Waals contact. The real-world version of these representations are CPK models (Corey-Pauling-Koltun). Although occupied spaces are shown well here, the bond distances and angles are more difficult to appreciate. |
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In the age of computers we can do more (bottom row). For example, we can show a volume occupied by electrons in such a way that 95% (4, bottom right) or 50% (5, bottom center) of the total electron density is inside the enclosed space. The bigger "blob" is indeed very similar to the space-filling model (3, top center), although the demarcation line between he contributing atoms is not apparent. But these molecular "blobs" are not really gray and boring! For example, we can map out (in color) differences in electron density on the surface of the molecule. Imagine that we measure the electrostatic interaction between a positive charge (a probe) and the electron cloud at the surface of this molecular "skin." There are places of higher electron density (stronger attraction of the probe) shown in red (6) and places where the electron density is low (weaker attraction or repulsion of the probe) shown in blue. This picture should not be a surprise. You know that carbon is more electronegative than hydrogen, and therefore, the shared electrons will be pulled toward the center. The interactive model on the right (6) uses different colors to show the same charge separation in a little more crude manner. And to remind you again that we are dealing here with 3-D objects, we offer another "flipping" demonstration: |
| Molecular Gallery | Last updated 06/07/07 | Copyright 1997-2008 |
