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Molecular Geometry

In this section, we will study the valence shell electron pair repulsion theory and the basis for predicting the shapes of some structures.

Valence Shell Electron Pair Repulsion Theory

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.

Molecular geometry of a molecule is the directional orientation of the bonded pairs around the central atom, excluding the unshared electron pairs.

Let's look at the practical significance of this theory through some examples.

Example 5-3

Predict the molecular geometry of the CO2 molecule.



Carbon, obviously the central atom, has four electrons in its valence shell. In order to complete the octet, carbon requires four more electrons or two pairs of electrons. Oxygen atom has six electrons in its valence shell. In order to complete its octet, each oxygen atom requires two electrons or one pair of electrons. The Lewis structure of CO2 should look like the figure shown below. Note that the valence electrons of the carbon atom are denoted by asterisks (*) and that of the oxygen are denoted by dots.


image\24582 Ch 5.jpg


The molecule keeps this linear shape, so that the electrons are placed far apart as predicted by the VSEPR theory. Also notice that the carbon-oxygen bonds are double bonds. Hence the CO2 molecule is linear in shape.


Example 5-4

Predict the shape of the BeF2 molecule.



Beryllium has two electrons in its outer most shell. In this case, there is an exception to the octet rule, because in the formation of the molecule, only two pairs (four electrons) are present in the valence shell of beryllium. Each fluorine atom requires one electron to attain the complete octet. Take a look at the structure shown below. Again for clarity, beryllium electrons are denoted by asterisks and fluorine electrons are denoted by dots.


image\Ch 5 page 72 first graphic.png


Example 5-5

Predict the shape of the water (H2O) molecule.



When dealing with bond formation with the possibility of existence of lone pairs, we have to take that into consideration. You will see that in this example. The oxygen atom has six valence electrons, and the hydrogen atom has one valence electron. So in the structural formation, there are two lone pairs in the central oxygen atom. How do you think this will change the shape of the molecule? Well, because of the lone pairs, the molecule will be bent, rather than linear. The structure is shown below.


image\24601 Ch 5.jpg


Problem 5-3
Predict the geometry of the following molecules.
  1. BeCl2
  2. NH3
  3. SiCl4
  4. BF3
  5. CH4
  1. linear
  2. trigonal pyramidal
  3. tetrahedral
  4. trigonal planar
  5. tetrahedral

Some Common Geometries And Their Characteristics

image\Ch 5 page 73 table 5-1.png

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