Predicting the Shapes of Molecules

Distribute the remaining electrons, first to terminal atoms. 

Notice that you could have formed the double bond with either of the other two oxygen atoms. 

Since the three Lewis structures are equally correct, write the three structures as resonance structures. 

Write the Lewis structure for the N02 ion. Include resonance structures. 

Lewis theory, in combination with valence shell electron pair repulsion (VSEPR) theory, can be used to predict the shapes of molecules. VSEPR theory is based on the idea that electron groups—lone pairs, single bonds, or multiple bonds—repel each other. This repulsion between the negative charges of electron groups on the central atom determines the geometry of the molecule. For example, consider CO2, which has the Lewis structure  

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To help us predict the shapes of molecules, we use the generic VSEPR formula ... 

Example the n = 2 shell of Period 2 atoms, valence-shell electron-pair repulsion model (VSEPR model) A model for predicting the shapes of molecules, using the fact that electron pairs repel one another. 

VSEPR theory works best when predicting the shapes of molecules composed of a central atom surrounded by bonded atoms and nonbonding electrons. Some of the possible shapes of molecules that contain a central atom are given in Figure 7.11, along with the chemical formulas of molecules that have that shape. 

The shape of a molecule has quite a bit to do with its reactivity. This is especially true in biochemical processes, where slight changes in shape in three-dimensional space might make a certain molecule inactive or cause an adverse side effect. One way to predict the shape of molecules is the valence-shell electron-pair repulsion (VSEPR) theory. The... 

Lone pairs on the central atom of a molecule also affect the shape of the molecule. To help us predict the shapes of molecules, we use a generic VSEPR formula AX Em to identify the different combinations of atoms and lone pairs attached to the central atom. We let A represent a central atom, X an attached atom, and E a lone pair. For example, the BF3 molecule, with three attached fluorine atoms and no lone pairs on B, is an example of an AX3 species. The sulfite ion, S()32- (18), is an example of an AX3E species. 

Atoms are bound into molecules by shared pairs of electrons. Electrons dislike each other because like charges repel each other. Therefore, whether they are lone pairs of electrons or bonding pairs of electrons, electron pairs try to get as far apart in space as is geometrically possible. There is a fancy name that summarizes these simple ideas the VSEPR theory, which stands for Valence Shell Electron Pair Repulsion Theory. Even though the VSEPR theory is founded on fundamentally simple ideas, it is a tremendously powerful tool for predicting the shapes of molecules. 

By predicting the shapes of molecules, you can predict their polarity. 

Molecular shape is defined by the locations of a molecule s atoms, not from the location of its electrons, because, experimentally, the scientist can determine the location of the atoms by X-ray diffraction and other studies. We can predict the shapes of molecules from their formulas by using a few simple rules. The ability to make such predictions enables us to understand better the properties of the substances. 

This is used to predict the shape of molecules. In order to be able to predict the geometry (or shape) of a molecule several simple steps are required. For example, consider the case of BrF-j. In this case we have indicated by... 

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. 

The tetrahedral sp3 hybridisation is very common in second row elements. However, in the third row elements there is the possibility of using one or more of the d sub-orbitals that are now available. We used the electron pair repulsion theory in Chapter 4 to predict the shape of molecules of general formula ABW, where n varied from two to five. If n equals five or six, what do you think... 

Depending on the types and numbers of atoms involved, the molecules can form many different shapes. The valence shell electron pair repulsion (VSEPR) theory suggests that the shape a molecule forms is based on the valence electrons surrounding the central atom. In this lesson, we will explore the process of predicting the shapes of molecules. 

In this chapter we have explored the structure of organic compounds. This is important since structure determines reactivity. We have seen that weak bonds are a source of reactivity. Strong bonds are made by good overlap of similar-sized orbitals (same row on periodic table). Bends or twists that decrease orbital overlap weaken bonds. Lewis structures and resonance forms along with electron flow arrows allow us to keep track of electrons and explain the changes that occur in reactions. VSEPR will help us predict the shape of molecules. Next we must review how bonds are made and broken, and what makes reactions favorable. Critical concepts and skills from this chapter are ... 

This valence-bond method, which takes account of the possibility of orbital hybridization, provides us with a useful procedure for predicting the shapes of molecules. The method has been used here to predict the shapes of molecules containing carbon, but the method can also be applied to other types of molecules. The procedure helps us to predict the angles between the bonds emanating from each atom, and thus allows us to visualize the entire structure of the molecule. 

VSEPR (valence-shell electron-pair repulsion) A method of predicting the shape of molecules. In this theory one atom is taken to be the central atom and pairs of valence electrons are drawn round the central atom. The shape of the molecule is deter-... 

Valence shell electron pair repulsion theory (VSEPR) can be used to predict the shapes of molecules. According to this theory, the geometry of a molecule is such that the valence-electron pairs of the central atom are kept farthest apart to minimize the electron repulsions. Again, you have to view molecules in terms of Lewis structure so that the shape of the molecules can be predicted with the VSEPR theory. 

Pairs of electrons, childish as they are, behave in a similar manner — those negative charges want to be as fcir apart as possible. The valence shell electron pair repulsion (VSEPR) theory is used to predict the shapes of molecules based on the idea that similarly charged electron pairs want to be as far apart from one another as possible. Though it looks like V-S-E-P-R, the theory name is most commonly pronounced vesper. ... 

We can generalize the steps we follow in using the VSEPR model to predict the shapes of molecules or ions ... 

Lewis structures can be used to explain and predict the shapes of molecules. The basic assumption is that, if the core of an atom is effectively spherical (as for most atoms it is), groups of electrons in the Lewis shell (single bond pairs, double-bond quartets, fractional bond pairs, etc.) get as far apart as possible. Thus, two groups take up a linear arrangement, three a trigonal-planar one, four tetrahedral, five trigonal-bip5Tamidal, six octahedral, and so on. 

The VSEPR method can also be used to predict the shapes of molecules containing multiple bonds if we assume that all of the electrons of a multiple bond act as though they were a single unit and, therefore, are located in the region of space between the two atoms joined by a multiple bond. 

In Chapter 9, we discussed bonding in terms of the Lewis theory. Here we will stndy the shape, or geometry, of molecnles. Geometry has an important influence on the physical and chemical properties of molecules, such as density, melting point, boihng point, and reactivity. We will see that we can predict the shapes of molecules with considerable accuracy using a simple method based on Lewis structures. 

Use VSEPR theory to predict the shapes of molecules and polyatomic ions. 

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