Shapes of molecules and polyatomic ions

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VSEPR stands for Valence Shell Electron Pair Repulsion and these electron pair repulsions are responsible for the shapes of molecules and polyatomic ions, such as NH+. 

Inorganic and physical chemistry Shapes of molecules and polyatomic ions... 

The writing of Lewis formulas is an electron bookkeeping method that is useful as a first approximation to suggest bonding schemes. It is important to remember that Lewis dot formulas only show the number of valence electrons, the number and kinds of bonds, and the order in which the atoms are connected. They are not intended to show the three-dimensional shapes of molecules and polyatomic ions. We will see in Chapter 8, however, that the three-dimensional geometry of a molecule can be predicted from its Lewis formula. 

The valence-shell electron-pair repulsion (VSEPR) theory is a theory used to predict probable shapes of molecules and polyatomic ions based on the mutual repulsions of electron pairs found in the valence shell of the central atom in the structure. 

The properties of molecules and polyatomic ions are determined to a great extent by their shapes. Incompletely filled electron shells and unshared pairs of electrons on the central element are very important. 

Predict the shapes of the following molecules and polyatomic ions. 1 a. NH2CI c. NO3... 

You now know how to draw Lewis structures for molecules and polyatomic ions. You can use them to determine the number of bonding pairs between atoms and the number of lone pairs present. Next, you will learn to describe molecular structure and predict the angles in a molecule, both of which determine the three-dimensional molecular shape. 

Special Topic 3.1 describes how the shape of ethanol molecules allows them to attach to specific sites on nerve cell membranes and slow the transfer of information from one neuron to another. Special Topic 5.2 describes how the shapes of the molecules in our food determine whether they taste sweet or bitter. You will find out in Special Topic 17.2 that the fat substitute Olestra is indigestible because it does not fit into the enzyme that digests natural fat. The purpose of this section is to show you how to use Lewis structures to predict three-dimensional shapes of simple molecules and polyatomic ions. Let s start with a review of some of the information from Section 3.1, where this topic was first introduced. 

In Section 7-6 we described resonance formulas for molecules and polyatomic ions. Resonance is said to exist when two or more equivalent Lewis formulas can be written for the same species and a single such formula does not account for the properties of a substance. In molecular orbital terminology, a more appropriate description involves delocalization of electrons. The shapes of molecular orbitals for species in which electron delocalization occurs can be predicted by combining all the contributing atomic orbitals. 

Hybridization of atomic orbitals. To account for the bonding in simple diatomic molecules like HF, we picture the direct overlap of 5 and p orbitals of isolated atoms. But how can we account for the shapes of so many molecules and polyatomic ions through the overlap of spherical s orbitals, dumbbell-shaped p orbitals, and cloverleaf-shaped d orbitals ... 

The VSEPR model reliably predicts the geometry of many molecules and polyatomic ions. Chemists use the VSEPR approach because of its simplicity. Although there are some theoretical concerns about whether electron-pair repulsion actually determines molecular shapes, the assumption that it does leads to useful (and generally reliable) predictions. Example 4.1 illustrates the application of VSEPR. 

Most molecules and polyatomic ions do not have flat, two-dimensional shapes like those implied by the molecular Lewis structures of Table 4.3. In fact, the atoms of most molecules and polyatomic ions form distinct three-dimensional shapes. Being able to predict the shape is important because the shape contributes to the properties of the molecule or ion. This can be done quite readily for molecules composed of representative elements. 

The shapes of many molecules and polyatomic ions can be predicted by using the valence-shell electron-pair repulsion theory (VSEPR). According to the VSEPR theory, electron pairs in the valence shell of the central atom of a molecule or ion repel one another and become arranged so as to maximize their separation distances. The resulting arrangement determines the molecular or ionic shape when one or all of the electron pairs involved form bonds between the central atom and other atoms. 

The molecule or polyatomic ion adopts the shape that minimizes the repulsion between the bonding and lone pairs of electrons. The shapes of the molecules and polyatomic ions are therefore determined by the electron pairs rather than by the atoms. 

We have seen earlier (Chapter 4) that the valence shell electron pair repulsion theory (VSEPR theory) can be very usefully applied to explain the shapes of simple covalent molecules and polyatomic ions built around a central atom. 

Electron-dot formulas can be used to diagram the sharing of valence electrons in molecules and polyatomic ions. The presence of multiple bonds can be identified, and possible resonance structures can be drawn. From the electron-dot formulas, we can predict the three-dimensional shapes and polarities of molecules. Then we examine how the different attractive forces between the particles of ions and molecules influence their physical properties, such as melting and boiling point. Finally, we discuss the physical states of solids, liquids, and gases and describe the energy involved in changes of state. 

Strength The VSEPR model enables us to predict the shapes of many molecules and polyatomic ions. 

You can use the steps below to help you predict the shape of a molecule (or polyatomic ion) that has one central atom. Refer to these steps as you work through the Sample Problems and the Practice Problems that follow. 

Use the VSEPR model to predict the electron arrangement and shape of a molecule or polyatomic ion from its formula, giving each bond angle approximately, Examples 3.1, 3.2, and 3.3. 

The VSEPR theory has its roots in the observation prior to 1940 that isoelectronic molecules or polyatomic ions usually adopt the same shape. Thus BF3, B03 C03, COF2 and NO3 are ail isoelectronic, and they all have planar triangular structures. As developed in more recent years, the VSEPR theory rationalises molecular shapes in terms of repulsions between electron pairs, bonding and nonbonding. It is assumed that the reader is familiar with the rudiments of the theory excellent expositions are to be found in most inorganic texts. 

An advantage of VSEPR is its foundation upon Lewis electron-pair bond theory. No mention need be made of orbitals and overlap. If you can write down a Lewis structure for the molecule or polyatomic ion in question, with all valence electrons accounted for in bonding or nonbonding pairs, there should be no difficulty in arriving at the VSEPR prediction of its likely shape. Even when there may be some ambiguity as to the most appropriate Lewis structure, the VSEPR approach leads to the same result. For example, the molecule HIO, could be rendered, in terms of Lewis theory as ... 

T-shaped A term used to describe the molecular geometry of a molecule or polyatomic ion that has three atoms bonded to a central atom and two unshared pairs on the central atom (AB3U2). 

Molecules, from simple diatomic ones to macromolecules consisting of hundreds of atoms or more, come in many shapes and sizes. The term molecular geometry is used to describe the shape of a molecule or polyatomic ion as it would appear to the eye (if we could actually see one). For this discussion, the terms molecule and molecular geometry pertain to polyatomic ions as well as molecules. 

You learned in Chapter 12 that atoms in molecular compounds and polyatomic ions are held together by covalent bonds. Lewis diagrams show, in two dimensions, how the atoms are connected. However, Lewis diagrams do not show how the atoms are arranged n three dimensions-the actual shape of the molecule. In this chapter you will learn how the distribution of atoms leads to the structure and shape of molecules. It begins with the Lewis diagram, and in case it has been a while since you studied Lewis diagrams, we will review them briefly. Important terms are printed in italics. 

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