Molecules predicting shapes

Sei e-Tfst 3.3A (a) Give the VSEPR formula of an NH3 molecule. Predict (b) its electron arrangement and (c) its shape. 

Figure 4.20 Predicted shapes of molecules containing multiple bonds.

The VSEPR notation for the Cl2F+ ion is AX2E3. According to Table 11.1, molecules of this type exhibit an angular molecular geometry. Our next task is to select a hybridization scheme that is consistent with the predicted shape. It turns out that the only way we can end up with a tetrahedral array of electron groups is if the central chlorine atom is sp3 hybridized. In this scheme, two of the sp3 hybrid orbitals are filled, while the remaining two are half occupied. 

The best arrangement of a given number of electron pairs is the one that minimizes the repulsion among them. This simple idea is the basis of the VSEPR. This model is used to predict shapes of molecules. 

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. 

Table 10 The shape of AH2 molecules predicted by Walsh rules"...

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.

Figure 6.1 The predicted shapes of simple molecules, anions and cations...

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. 

Table 11 The shape of AB2 molecules predicted by Walsh rules ...

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. 

Consider the given Lewis structure for BrFj. What is the predicted shape for the molecule as a whole p/ -p . ... 

Connection between shape and electronic structure Picturing the shape and energy of molecular orbitals in simple molecules Predicting the locations of lone pairs and empty orbitals -... 

Why do so many AB molecules have shapes related to those shown in Figure 9.3, and can we predict these shapes When A is a representative element (one from the s block or p block of the periodic table), we can answer these questions by using the valence-shell electron-pair repulsion (VSEPR) model. Although the name is rather imposing, the model is quite simple. It has useful predictive capabilities, as we will see in Section 9.2. 

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