Carbon dioxide VSEPR

Three triatomic molecules, water, ozone, and carbon dioxide, provide particularly strong experimental evidence to support the VSEPR ... 

A) The carbon of carbon dioxide has two double bonds. Because there are no unshared pairs of electrons on the central carbon atom, VSEPR theory predicts a linear molecular geometry (type AX2). 

The application of VSEPR theory to triatomic molecules is exemplified by considering water, carbon dioxide, xenon difluoride and a trio of connected species the nitronium ion, N02+, nitrogen dioxide and the nitrite [or nitrate(III)] ion, N02. ... 

Multiple bonds are treated as a single unit in the VSEPR model. Formaldehyde is a trigonal planar molecule in which the electrons of the double bond and those of the two single bonds are maximally separated. A linear arrangement of atoms in carbon dioxide allows the electrons in one double bond to be as far away as possible from the electrons in the other double bond. 

Now, what about carbon dioxide Well, when we draw a Lewis structure of carbon dioxide, and consider VSEPR theory, we arrive at a linear molecule, as illustrated in Figure 7.15. Each C-O bond is polarized towards the oxygen. However, the two bond dipoles are pointing in exactly opposite directions, and cancel each other out. Therefore, carbon dioxide is a nonpolar molecule. 

When we are to determine how many electron groups that surround an atom, the Lewis structure can be of great help (see the previous section 2.23 Lewis structure). From the Lewis structure of a given molecule you can simply count how many bonds and lone pairs that surround an atom. That way you have the number of electron groups. The VSEPR theoiy tells us that these electron groups will be placed as far apart as possible. In the following example we will use the VSEPR theory to predict the molecular geometries of a water molecule and a carbon dioxide molecule. That way we will discover why a carbon dioxide molecule is linear and why a water molecule is V-shaped. 

We wish to predict the molecular geometries of a water molecule and a carbon dioxide molecule respectively. The VSEPR theory is our tool to solve this job and it tells us that the electron groups surrounding the central atom will be placed as far apart as possible. 

The VSEPR theory has thus served as a tool that enabled us to explain why a carbon dioxide molecule is linear and why a water molecule is V-shaped. The VSEPR theory is a simple and usable tool to predict geometries of molecules when the Eewis structure is already available giving us the number of electron groups. 

From the Lewis structure of the carbon dioxide molecule (Figure 2- 18a) it is seen that the carbon atom is surrounded by two electron groups (two double bonds). Two electron groups mean that there is a need for two identical orbitals 180° apart according to the VSEPR theory and Table 2- 1 on page 70. The carbon atom solves this problem by forming two identical so-called sp-hybrid orbital. As the name sp indicated these orbitals are made from one s-orbital and one p-orbital. 

How do we work out the shape of a molecule when it contains double, or even triple bonds We can still use VSEPR theory, but a double or triple bond is treated as though it were a single bond, that is, one bonding pair of electrons. Carbon dioxide has the structural formula ... 

In still other types of molecules, a central atom is surrounded by only two regions of electron density. Figure 1.10 shows Lewis structures and ball-and-stick models of carbon dioxide (CO2) and acetylene (C2H2). As with double bonds, VSEPR treats triple bonds as one region of electron density. 

The VSEPR model can also be applied to molecules with multiple bonds. In this case, each multiple bond is treated as though it were a single electron pair. (All pairs of a multiple bond are required to be in approximately the same region.) To predict the geometry of carbon dioxide, for example, you first write the electron-dot formula. 

You have two double bonds, or two electron groups, about the carbon atom, and these are treated as though there were two pairs on carbon. Thus, according to the VSEPR model, the bonds are arranged linearly and the geometry of carbon dioxide is linear. 

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