Lone pair arrangements, hydrogen bonds

If we examine the other central atom, the oxygen with the attached hydrogen, we observe the presence of two lone pairs and two bonds. The presence of these pairs and bonds, which total four, means that the electron-group geometry is tetrahedral. This arrangement has sp3 4 5 6 hybridization. Since there are two lone pairs, the molecular geometry is bent. 

In Chapter 3, you were told that carhon atoms usually have four bonds, oxygen atoms usually have two bonds and two lone pairs, and hydrogen atoms form one bond. Using guidelines such as these, we can predict that there are two possible arrangements of the atoms of C2HgO. 

Figure 11.10 Lewis structures of water (H20). (a) shows two possible configurations of water, but only H-O-H satisfies the electronic requirements of the oxygen atom, (b) shows three possible bond distributions for this structure, but only one (with a single bond to each of the hydrogens and two lone pairs on the oxygen) meets the requirements of all three atoms, (c) shows the bent structure of H-O-H which follows from the need to separate the two lone pairs and two single bonds as far as possible in the three-dimensional molecule, (d) shows a space-filling version of this arrangement, where the oxygen is black and the two hydrogens white.

You re now finished with the simple part. One carbon atom in the structure still requires two additional electrons to fill its valence shell. The only way to fill this shell is to take the lone pair of electrons you added to one of the carbons and instead use it to create a double bond between two of the carbons. You then need to move the hydrogens around to ensure that each carbon has a total of four bonds. Only one arrangement of hydrogen atoms to the three carbons allows you to fill all the carbon valence shells, as you can see in the following figure ... 

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