The Valence Shell Electron Pair Repulsion Model
The geometry adopted by groups around an atom can be predicted by using a particularly simple model known as the valence shell electron pair repulsion model (VSEPR). This model is based on the simple idea formulated by Lewis and others that electrons tend to move as far away from one another as possible (like charges repel one another). Because electrons tend to pair with one another if their spins are opposed, we can modify the idea to say that pairs of electrons tend to adopt a geometry that minimizes repulsions. We have noted earlier that only the electrons in the valence shell are involved in bonding, so we only need to worry about those electrons.
Let us consider some examples. Carbon dioxide, a crucial part of photosynthesis, has the electron-dot structure
If we want to know the structure of this compound, we might first consider some possibilites. There are only two of them; either the three atoms lie on a straight line or they do not. That is, the compound can have only two different geometries--linear or bent. The geometry is determined by the tendency of the electron pairs around the central atom to minimize repulsions. How many pairs of electrons are there at the carbon? Clearly, the electron dot formula shows no lone pairs; it does show two sigma-bonded pairs and two 1-bonded pairs. It turns out that the 1-bonds do not influence the geometry, so the sigma-bonded (and lone pairs) are the sole determinants of geometry. Hence, only the two sigma-bonded pairs around the carbon of CO2 need be considered. How should two pairs of electrons be arranged around an atom in order to minimize the repulsions between them? The answer is clear--at the opposite ends of a straight line. Hence, we can conclude that CO2 is a linear molecule.
Now consider the nitrite ion. Reference to the electron-dot formula given above shows that there is one lone pair and two sigma-bonded pairs at the central nitrogen. How are three pairs of electrons arranged to minimize repulsions? Again, the answer is quite reasonable--at the corners of a triangle as shown in Figure 38. At the ends of two of the pairs are the oxygens; the other pair of electrons is the lone pair on the central nitrogen. Because the angle between the oxygens is less than 180°, the geometry of the ion is described as bent. Other geometries are summarized in Table 4. It is important to understand that the geometry is determined by the positions of the atoms, not all of the electron pairs. For example, even though the geometry of the electron pairs around the oxygen in water is tetrahedral, it is the position of the atoms that leads to the description of bent for the geometry of water.
Figure 38. The geometry of the nitrite ion.