VSEPR Theory Predicting Geometry

TABLE 1.2 COMPARISON OF BOND LENGTHS AND BOND ENERGIES FOR ETHANE, ETHYLENE, AND ACETYLENE 

24 Rank the indicated bonds in terms of increasing bond length  

Calculation of the steric number for methane, ammonia, and water. 

The steric number indicates the number of electron pairs (bonding and nonbonding) that are repeUing each other. The repulsion causes the electron pairs to arrange themselves in three-dimensional space so as to achieve maximal distance from each other. As a result, the geometry of the central atom will be determined by the steric number. This principle is called the t/alence hell electron air repulsion (VSEPR) theory. Let s take a closer look at the geometry of each of the compounds above. 

EXAMPLE STERIC NUMBER HYBRIDIZATION ARRANGEMENT OF ELECTRON PAIRS ARRANGEMENT OF ATOMS (geometry) 

---

A) The carbon of carbon dioxide has two double bonds. Because there are no unshared pairs of electrons on the central carbon atom, VSEPR theory predicts a linear molecular geometry (type AX2). 

Methane, CH, has four hydrogen atoms bonded to a central carbon atom. Ammonia, NH3, has three hydrogen atoms bonded to a central nitrogen atom. Using VSEPR theory, predict the molecular geometry of each compound. 

Figure 3.13 also shows the structure for CH20. Because VSEPR theory predicts a trigonal planar geometry for the double bond and the two electron pairs on the oxygen, we will treat it as spr hybridized. Again, the picture is similar to ethene, with one CO sigma bond and one CO pi bond, but with both unshared electron pairs in... 

The VSEPR theory predicts the three-dimensional shapes of molecules. It is based on simple electrostatics—electron pairs in a molecule will arrange themselves in such a way as to minimize their mutual repulsion. The steric number determines the geometry of the electron pairs (linear, trigonal pyramidal, tetrahedral, and so forth), whereas the molecular geometry is determined by the arrangement of the nuclei and may be less symmetric than the geometry of the electron pairs. 

VSEPR theory predicts that four valence shell electron pairs are directed toward the corners of a regular tetrahedron. That shape gives the maximum separation for four electron pairs around one atom. Thus, VSEPR theory predicts tetrahedral electronic geometry for an AB molecule that has no unshared electrons on A. There are no lone pairs of electrons on the central atom, so another atom is at each corner of the tetrahedron. VSEPR theory predicts a tetrahedral molecular geometry for each of these molecules. 

With the same kind of reasoning, VSEPR theory predicts that sulfite ion, 803 , has tetrahedral electronic geometry. One of these tetrahedral locations is occupied by the sulfur lone pair, and oxygen atoms are at the other three locations. The molecular geometry of this ion is trigonal pyramidal, the same as for other AB3U species. 

By similar reasoning, VSEPR theory predicts octahedral electronic geometry and octahedral molecular geometry for the PFg ion, which has six valence shell electron pairs and six F atoms surrounding one P atom. 

Solution. The structure of H2O is shown below, where the labels on the H atoms are imaginary. VSEPR theory predicts tetrahedral electron geometry with a bent molecular geometry and bond angles less than 109.5°. The complete set of symmetry operations is , C2, f7y, and (7 2- The multiplication table is shown below. In this symmetry group, each operation is its own inverse. 

In Chapter 15 we shall learn how the nitronium ion, N02, forms when concentrated nitric and sulfuric acids are mixed, (a) Write a Lewis structure for the nitronium ion. (b) What geometry does VSEPR theory predict for the N02 ion (c) Give a species that has the... 

Predicting Geometry Using VSEPR Theory Predict the electron and molecular geometry of PCI3. Predict the electron and molecular geometry of the [N03] ion. ... 

When carbon forms a double bond and two single bonds, there are three electron groups around each carbon atom, and VSEPR theory predicts a trigonal planar geometry. 

H — S — H, VSEPR theory predicts a tetrahedral electron-group geometry, which in turn suggests a tetrahedral bond angle—that is, 109.5°. However, by modifying this initial VSEPR prediction to accommodate lone-pair-lone-pair and lone-pair-bond-pair repulsions (see page 443), the predicted bond angle is less than 109.5°. 

When VSEPR theory is used to predict molecular geometries, double and triple bonds are treated identically to single bonds as a single electron group, i.e. as a single place where you can find electrons. 

The Cl—F and Cl—Cl bonds in the cation are then formed by the overlap of the half-filled sp3 hybrid orbitals of the central chlorine atom with the half-filled p-orbitals of the terminal Cl and F atoms. Thus, by using sp3 hybridization, we end up with the same bent molecular geometry for the ion as that predicted by VSEPR theory (when the lone pairs on the central atom are ignored)... 

The result here is quite satisfactory because XeF4 does in fact exhibit square planar geometry. It is worth noting, however, that a square planar shape for XeF4 is also predicted by VSEPR theory. Despite the fact that the molecular orbital method has made some inroads as of late, VSEPR is still the best approach available for rationalizing the molecular geometries of noble gas compounds. 

Molecular geometry, the arrangement of atoms in three-dimensional space, can be predicted using the VSEPR theory. This theory says the electron pairs around a central atom will try to get as far as possible from each other to minimize the repulsive forces. 

---