Shapes of molecules and ions

Table 1.4 Shapes of Molecules and Ions from VSEPR Theory...

The shapes of molecules and ions are explained by the valence shell... 

Describe the models that chemists use to describe the structure and shape of molecules and ions, and assess their value in terms of the information they communicate. 

A most important principle to recognise is that the shapes of molecules and ions formed by non-transitional elements are determined mainly by the number of electron pairs round the central atom and by the repulsions between them. Lone-pair-lone-pair repulsion is strongest, lone-pair-bond-pair next and bond-pair-bond-pair weakest. 

VSEPR theory (10) Valence shell electron pair repulsion theory, which predicts the three-dimensional shape of molecules and ions based on the arrangement of electron pairs (nonbonding pairs and bonds) about the central atom. 

TABLE 1.3 SHAPES OF MOLECULES AND IONS FROM VSEPR THEORY ... 

Finally, in addition to simply representing a pair of shared electrons, a chemical bond has structural implications as well. Because electrons are negatively charged, when there are several distinct bonds, they will tend to be physically separated from each other. This idea is the basis for a method to predict the geometry of molecules called the Valence Shell Electron Pair Repulsion (VSEPR) theory. Using this theory, the general shape of molecules and ions can be predicted. 

The electron pairs around a central atom will become oriented in space to get as far away from one another as possible. Thus, two pairs will be oriented with one pair on each opposite side of the central atom. Three pairs will form a triangle around the central atom, and four pairs will be located at the comers of a regular tetrahedron with the central atom in the center (see i Eigure 4.5 and > Eigure 4.6). The VSEPR theory can be used to predict shapes of molecules and ions with five or more pairs on the central atom, but we will not go beyond four pairs in this book. 

Valence-shell electron-pair repulsion (VSEPR) model predicts the shapes of molecules and ions by assuming that the valence-shell electron pairs are arranged about each atom so that electron pairs are kept as far away from one another as possible, thus minimizing electron-pair repulsions. (10.1) van der Waals forces a general term for those intermolecular forces that include dipole-dipole and London forces. (11.5) van der Waals equation an equation similar to the ideal gas law, but includes two constants, a and b, to account for deviations from ideal behavior. (5.8)... 

The shapes of molecules and ions can be predicted by the valence shell electron pair repulsion theory (VSEPR). If the Lewis structure is drawn for a molecule or a polyatomic ion, the shape of this molecule or ion can be predicted using this theory. 

VSEPR theory can also be used to explain the shapes of molecules or ions that contain a double or triple bond. A double or triple bond has the same effect as a single bond because all the bonding pairs of electrons are located between the two atoms forming a covalent bond. A double or triple bond is therefore counted as one bonding pair (one electron domain) when predicting the shapes of molecules and ions. 

The shapes of molecules and ions are determined mainly by the number of electron pairs located around the central atom of the molecule or ion, and the effects of any lone pairs of electrons on the bond angles. Another factor that affects the bond angle is the electronegativity of the central atom. 

Shapes of Molecules and Ions of Representative Elements with No Unshared Pairs of Electrons on Central Atom... 

Chapter 10, Properties of Solids and Liquids, introduces electron-dot formulas for molecules and ions with single and multiple bonds as well as resonance structures. Electronegativity leads to a discussion of the polarity of bonds and molecules. Electron-dot formulas and VSEPR theory illustrate covalent bonding and the three-dimensional shapes of molecules and ions. The attractive forces between particles and their impact on states of matter and changes of state are described. Combining Ideas from Chapters 8, 9, and 10 follows as an interchapter problem set. 

---