Molecular Structure The VSEPR Model

The structures of molecules play a very important role in determining their chemical properties. As we will see later, structure is particularly important for biological molecules a slight change in the structure of a large biomolecule can completely destroy its usefulness to a cell or may even change the cell from normal to cancerous. 

Note that there are two pairs of electrons around the berylUum atom. What arrangement of these electron pairs allows them to be as far apart as possi- 

This is the maximum possible separation for two electron pairs. Once we have determined the optimal arrangement of the electron pairs around the central atom, we can specify the molecular structure of BeCl2—that is, the positions of the atoms. Since each electron pair on beryllium is shared with a chlorine atom, the molecule has a linear structure with a bond angle of 180 degrees  

Here the boron atom is surrounded by three pairs of electrons. What arrangement will minimize the repulsions The electron pairs are farthest apart at angles of 120 degrees  

Since each electron pair is shared with a fluorine atom, the molecular structure is 

Take the sum of the lone pairelectrons and one-half the shared electrons. This is the number of valence electrons assigned to the atom in the molecule. 

Subtract the number of assigned electrons from the number of valence electrons on the free, neutral atom to obtain the formal charge. 

Give possible Lewis structures for XeOs, an explosive compound of xenon. Which Lewis structure or structures are most appropriate according to the formal charges  

For XeOs (26 valence electrons) we can draw the foUowing possible Lewis structures (formal charges are indicated in parentheses)  

Based on the ideas of formal charge, we would predict that the Lewis structures with the lower values of formal charge would be most appropriate for describing the 

On the other hand, some molecules exhibit a linear structure (all atoms in a line). An example is the CO2 molecule. 

Note that a linear molecule has a 180° bond angle. 

A third type of molecular structure is illustrated by BF3, which is planar or flat (all four atoms in the same plane) with 120° bond angles. 

The name usually given to this structure is trigonal planar structure, although triangular might seem to make more sense. 

In the next section we will discuss these various molecular structures in more detail. In that section we will learn how to predict the molecular structure of a molecule by looking at the molecule s Lewis structure. 

OBJECTIVE To learn to predict molecular geometry from the number of electron pairs. 

Whenever two pairs of electrons are present around an atom, they should always be placed at an angle of 18(T to each other to give a linear arrangement. 

In this same vein, note the difference between a correct, or valid, Lewis structure and an electronic structure that accurately accounts for a molecule s observed properties. A valid Lewis structure is one that obeys the rules we have established for Lewis structures. However, this Lewis structure may or may not give an accurate picture of the molecule and its properties. Experiments must be carried out to make the final decisions on the correct description of the bonding in a molecule or polyatomic ion.  

The origin of the repulsions among electron pairs probably results more from the operation of the Pauli exclusion principle than from electrostatic effects, but we will not be concerned with that in this text. 

BeClj has only four electrons around Be and is expected to be very reactive with electron-pair donors. 

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