Predicting Molecular Shapes

Follow the four-step procedure that helps to predict molecular shape. Use Table 4.2 for names of the electron-group arrangements and molecular shapes. 

The first thing you must be able to do in order to predict molecular shapes is to draw an electron-dot formula, so we ll tackle that subject first Including H, there are 16 active nonmetals for which you should know the numbers of valence electrons in the uncombined atoms Except for H (which has only one s electron), these elements are all found to the right of the diagonal in the p block of the periodic table (see inside front cover) Each atom has two v electrons in its valence shell, the number ofp electrons is different for different atoms (Basically, we are uninterested in metals here, metals rarely form predominantly covalent bonds, but tend to form ionic bonds ignore the noble gases, with an already filled s-yi6 unreactive )... 

The electron-dot structures described in Sections 7.6 and 7.7 provide a simple way to predict the distribution of valence electrons in a molecule, and the VSEPR model discussed in Section 7.9 provides a simple way to predict molecular shapes. Neither model, however, says anything about the detailed electronic nature of covalent bonds. To describe bonding, a quantum mechanical model called valence bond theory has been developed. 

Students will predict molecular shapes and construct molecular models from molecular formulas. 

For a given compound, write down the chemical symbols of its constituent atoms and position them to show their rough layout in the molecule (Lewis theory alone cannot predict molecular shape). 

For more than a century, scientists have intensely studied the geometry of compounds. Swiss chemist Alfred Werner (1866-1919) won the Nobel Prize in 1913 for his pioneering work predicting the shapes of mole-eules. Since those early studies, scientists have developed rules and guidelines based upon physical laws that predict molecular shapes. 

In attempting to predict molecular shapes, it is often useful to consider the oversimplified vi of molecules with independent electron orbitals (e.g., s andp orbitals). In the case of methane (CH4), the greatest distance in space that can separate the four carbon-hydrogen bonds around the central carbon atom occurs when the bonds are pointed at the corners of a tetrahedron. When the bonds are pointed toward the corners of a tetrahedron the bond angles are 109.5° and the molecule is said to be a tetrahedral molecule. 

A Lewis Structure Can Help Predict Molecular Shape... 

If you missed 35, go to VSEPR Theory—Predicting Molecular Shapes, page 280. 

If you answered incorrectly, review VSEPR Theory—Predicting Molecular Shape, page 280. 

The electron pairs around the central atom of the molecule arrange themselves to minimize electronic repulsion. This means that the electron pairs arrange themselves so that they can be as far as possible from each other. We may use this fact to predict molecular shape. This approach is termed the nalence shell electron pair repulsion (VSEPR) theory. 

SAMPLE PROBLEM 10.6 Predicting Molecular Shapes with Two, Three,... 

Predicting Molecular Shapes with More Than One Central Atom... 

Describe the five electron-group arrangements and associated molecular shapes, predict molecular shapes from Lewis structures, and explain deviations from ideal bond angles ( 10.2) (SPs 10.6-10.8) (EPs 10.25-10.49)... 

Knowledge Required (1) The inteqiretation of Lewis structures. (2) The valence-shell-electron-pair-repulsion (VSEPR) model for predicting molecular shape. 

How does the VSEPR model take into account the repulsion of electron pairs in predicting molecular shape ... 

The VSEPR model, simple as it is, does a surprisingly good job at predicting molecular shape, despite the fact that it has no obvious relationship to the filling and shapes of atomic orbitals. For example, based on the shapes and orientations of the 2s and 2p orbitals on a carbon atom, it is not obvious why a CH4 molecule should have a tetrahedral geometry. How can we reconcile the notion that covalent bonds are formed from overlap of atomic orbitals with the molecular geometries that come from the VSEPR model ... 

---