Carbon dioxide molecular geometry

We can combine our knowledge of molecular geometry with a feel for the polarity of chemical bonds to predict whether a molecule has a dipole moment or not The molec ular dipole moment is the resultant of all of the individual bond dipole moments of a substance Some molecules such as carbon dioxide have polar bonds but lack a dipole moment because their geometry causes the individual C=0 bond dipoles to cancel... 

For example, let s determine the electron-group and molecular geometry of carbon dioxide, C02, and water, H20. At first glance, you might imagine that the geometry of these two compounds would be similar since both have a central atom with two groups (atoms) attached. However, let s see if that is true. 

This determination of the molecular geometry of carbon dioxide and water also accounts for the fact that carbon dioxide does not possess a dipole and water has one, even though both are composed of polar covalent bonds. Carbon dioxide, because of its linear shape, has partial negative charges at both ends and a partial charge in the middle. To possess a dipole, one end of the molecule must have a positive charge and the other a negative end. Water, because of its bent shape, satisfies this requirement. Carbon dioxide does not. 

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). 

In these two cases the dipole arrows cancel each other out because of the shape of the molecules. The linear shape of the molecule of carbon dioxide puts the dipole arrows in opposite directions to counterbalance each other. The same holds true for the tetrahedral molecular geometry found in carbon tetrachloride. Despite having polar bonds, these two molecules are nonpolar. There is no overall dipole moment in these molecules because the dipole arrows are of the same magnitude but lie in opposite directions in the molecule. This counterbalance causes the molecule to be nonpolar. 

Nickel is required for the synthesis of active urease in plant and other cells. The enzyme catalyzes the hydrolysis of urea to carbon dioxide and ammonia, via the intermediate formation of carbamate ion (equation 46). The molecular weight has been redetermined recently as 590 000 30 000, with six subunits. Each subunit has two nickel centres and binds one mole of substrate. The activity of the enzyme is directly proportional to the nickel content, suggesting an essential role for nickel in the enzyme. Several approaches, including EXAFS measurements, suggest that histidine residues provide some ligands to nickel, and that the geometry is distorted octahedral. There appears to be a role for a unique cysteine residue in each subunit out of the 15 groups present. Covalent modification of this residue blocks the activity of the enzyme. 

Enzymes are proteins that act as catalysts for specific biochemical reactions in living systems. The reactants in enz)une-catalyzed reactions are called substrates. Thousands of vital processes in our bodies are catalyzed by many distinct enzymes. For instance, the enzyme carbonic anhydrase catalyzes the combination of CO2 and water (the substrates), facilitating most of the transport of carbon dioxide in the blood. This combination reaction, ordinarily uselessly slow, proceeds rapidly in the presence of carbonic anhydrase a single molecule of this enzyme can promote the conversion of more than 1 million molecules of carbon dioxide each second. Each enzyme is extremely specific, catalyzing only a few closely related reactions—or, in many cases, only one particular reaction—for only certain substrates. Modern theories of enzyme action attribute this to the requirement of very specific matching of shapes (molecular geometries) for a particular substrate to bind to a particular enzyme (Figure 16-19). 

Diatomic molecules containing atoms of different elements (for example, HCl, CO, and NO) have dipole moments and are called polar molecules. Diatomic molecules containing atoms of the same element (for example, H2, O2, and F2) are examples of nonpolar molecules because they do not have dipole moments. For a molecule made up of three or more atoms both the polarity of the bonds and the molecular geometry determine whether there is a dipole moment. Even if polar bonds are present, the molecule will not necessarily have a dipole moment. Carbon dioxide (CO2), for example, is a triatomic molecule, so its geometry is either linear or bent ... 

That way the Lewis structure for carbon dioxide has now told us that in this molecule the carbon atom is placed in the centre. Each oxygen atom is double bonded to the carbon atom and two lone pairs are attached to each oxygen atom. But from the Lewis structure we know nothing about the actual molecular geometry. 

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. 

Linear geometries are also relatively common. Carbon dioxide has the molecular formula CO2 and exists in a linear geometry with a 180-degree angle between the CO bonds. 

The molecular geometry of a molecule, how the atoms are arranged in three-dimensional space, is important for chemists to know because it often explains why certain reactions will or won t occur. In the area of medicine, for example, the molecular geometry of a drug may lead to side reactions. Molecular geometry also explains why water is a dipole (a molecule with a positive end and a negative end, like a magnet) and carbon dioxide is not. 

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