Bonding molecule, predicting shapes

Figure 4.20 Predicted shapes of molecules containing multiple bonds.

Use your knowledge of molecular shape and polar bonds to predict whether each molecule has an overall molecular polarity. 

Predict and sketch the three-dimensional shape of each multiple-bonded molecule. 

Hydrocarbon skeletons are built up from tetrahedral (sp3), trigonal planar (sp2),-or linear (sp) hybridized carbon atoms. It is not necessary for you to go through the hybridization process each time you want to work out the shape of a skeleton. In real life molecules are not made from their constituent atoms but from other molecules and it doesn t matter how complicated a molecule might be or where it comes from it will have an easily predictable shape. All you have to do is count up the single bonds at each carbon atom. If there are two, that carbon atom is linear (sp hybridized), if there are three, that carbon atom is trigonal (sp2 hybridized), and, if there are four, that carbon atom is tetrahedral (sp3 hybridized). 

Calculate the electronegativity difference for each bond in the molecules you built. Show partial charges. Based on the electronegativity difference and the predicted shape of each molecule, decide whether the molecule is polar or non-polar. 

Valence shell electron pair repulsion theory (VSEPR) provides a method for predicting the shape of molecules, based on the electron pair electrostatic repulsion. It was described by Sidgwick and Powell" in 1940 and further developed by Gillespie and Nyholm in 1957. In spite of this method s very simple approach, based on Lewis electron-dot structures, the VSEPR method predicts shapes that compare favorably with those determined experimentally. However, this approach at best provides approximate shapes for molecules, not a complete picture of bonding. The most common method of determining the actual stmctures is X-ray diffraction, although electron diffraction, neutron diffraction, and many types of spectroscopy are also used. In Chapter 5, we will provide some of the molecular orbital arguments for the shapes of simple molecules. 

Figure 1.13. Bond formation H2O molecule, a) Tetrahedral sp orbitals. (Jb) Predicted shape, showing unshared pairs H nuclei located for maximum overlap, (c) Shape and size.

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 molecular 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 shape (see Figure 1.12) causes the individual C=0 bond dipoles to cancel. 

The nitrogen atom in an ammonia molecule, NH3, forms three covalent bonds and in addition has a lone pair of electrons. A lone pair on a central atom must be considered in predicting a molecule s shape. 

Background Covalent bonding occurs when atoms share valence electrons. In the Valence Shell Electron Pair Repulsion (VSEPR) theory, the way in which valence electrons of bonding atoms are positioned is the basis for predicting a molecule s shape. This method of visualizing shape is also based on the molecule s Lewis structure. 

Label each MO with its group theoretical symbol state which orbitals are bonding, nonbonding, and antibonding fill in the electrons and determine the overall bond order and magnetism of the molecule. Also, sketch the predicted shape of each of the MOs. 

In conclusion, it seems fair to predict that, along with other C-H bond functionalization methods, C-H alkylation is revolutionizing the way chemists make carbon-carbon bonds in both simple and complex molecules, thereby shaping organic chemistry of the twenty-first century. 

Bonding theories are central to chemistry because they predict how atoms bond together to form compounds. They predict what combinations of atoms form compounds and what combinations do not. Bonding theories predict why salt is NaCl and not NaCl2 and why water is H2O and not H3O. Bonding theories also explain the shapes of molecules, which in turn determine many of their physical and chemical properties. The bonding theory you will learn in this chapter is called Lewis theory, named after G. N. Lewis (1875-1946), the American chemist... 

Bonding theories predict how atoms bond together to form molecules, and they also predict what combinations of atoms form molecules and what combinations do not. Likewise, bonding theories explain the shapes of molecules, which in turn determine many of their physical and chemical properties. 

The shape of a molecular particle plays a major role in determining the macroscopic properties of a substance. We examine this role in other chapters in this book. To better understand and predict the shape-property relationship, you should know what is responsible for molecular shape, and this is discussed in this section and the next. Discussion in these sections is limited to molecules having only single bonds. Molecules with multiple bonds are considered in Section 13.4. 

Multiple-bonded molecules. Draw electronic formulas and predict the shapes of the following molecules and ions (a) nitrosyl bromide, BrNO (b) ni-tryl chloride, CINO2 (c) thionyl chloride, SOCI2 (d) SO2 (e) phosgene, CI2CO. 

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