- VSEPR (Valence Shell Electron Repulsion Theory) is used to predict molecular geometry based on lone electron pairs repelling each other as much as possible
- They named the shape of the molecule according to pairs of bonding electrons
- The basic shape is determined by the number of electron pairs and bond angles
There are a lot of ways to categorize molecules, with the aim usually being to describe and predict the chemical and physical properties accurately. This is not only helpful in gaining a deeper understanding of the chemistry involved, but also in making assumptions about how molecules will behave under specific reaction conditions. Although the structure of molecules can get very complicated, fortunately, there is a fairly accurate and simple way to predict the geometry of amoleculeby using theVSEPR theory. We re going to get into how VSEPR theory works and how to use it, and provide you with a simple molecular geometry chart you can easily use in order to determine your molecule s basic shape as well as its bond angles.
What Is VSEPR?
VSEPR stands for Valence Shell Electron Pair Repulsion and is the most widely used model for predicting molecular geometry. The theory is underpinned by the principle of electron repulsion, which is the idea that electron pairs will naturally repel each other as far away as possible in a 3D space. This is true whether they re bonding electrons (shared between adjacent atoms) or lone pairs (not bonded to any atom and free in space). By situating themselves as far as possible, it decreases the energy of the molecule, and increases the stability. However, lone pairs of electrons tend to repel each other to a greater degree. By calculating the number of electron pairs around the central atom in the molecule, we can predict the molecular geometry with surprising accuracy. Using VSEPR, we can calculate bond angles and predict how molecules will react, as well as how stable they are.
How Do We Use VSEPR to Determine Molecular Geometry?
Using VSEPR involves a lot of steps, but most find it fairly intuitive and systematic. The general process for determining your molecule s shape is as follows:
- First, identify the central atom in the molecule. This is usually the atom with the highest valence, i.e., the greatest number of electrons and potential to bond with other atoms. The central atom is also usually the least electronegative atom, meaning it s more likely to share its electrons with other atoms.
- Next, count the valence electrons of the central atom (the number of electrons in its outer shell). You can do this by checking which group the element is in on a periodic table (the group is the column). This will give you the number of valence electrons.
- Once you have this number, you need to add one electron for every atom that s bonded to the central atom.
- The next step is to check if there s a charge on the central atom. If it s positive, you ll be removing electrons from your number, and if it s negative, you ll be adding them.
- The final calculation is to divide this electron number by 2 to obtain the number of electron pairs. You can then compare this number with the chart to determine the molecular geometry and get an idea of the bond angles.
We can also identify geometries by the steric number, which is the number of atoms bonded to the central atom. Lone pairs of electrons are indicated by bonds from the central atom with no terminal atom at the other end. As they are 3D representations, dotted lines indicate bonds coming forwards out of the plane and wedges indicate bonds going away from the viewer. Be sure to account for any lone pairs surrounding the central atom to get a more accurate prediction.
Let s illustrate this process with a simple example. Taking sulfur hexafluoride, or SF6, we can see that Sulfur is the central atom. This is because it has the most bonding potential out of all the atoms. We can see this from the 6 bonds it shares with fluorine atoms.
Using a periodic table, we can tell that sulfur is in group 6, therefore, it has 6 valence electrons.
The central sulfur atom has 6 fluorine atoms bonded to it. This gives us 6 more electrons, for a total of 12.
The molecule is neutral, so there is no charge on the sulfur atom. This leaves the electron number unchanged.
Finally, we divide the number by 2 to account for electron pairs. This gives us a final number of 6, with 0 lone pairs around the sulfur atom. Comparing this with the molecular geometry chart, we can see that the basic shape of SF6 is likely to be octahedral.
What Are the Limitations of VSEPR Theory?
While VSEPR is a convenient way to determine molecular geometry and works well in most cases, there are some limitations:
- We can t use the theory can t to predict the geometry of transitional metal complexes accurately. This is because these compounds have a rather unique electronic structure. The valence electrons aren t localized equally around the central atom. In addition, the d orbitals of the transition metal can interact with its ligands in different ways. VSEPR can t account for this. We must use alternative models, such as the ligand field theory and crystal field theory, to calculate the geometry.
- The VSEPR model doesn t consider bond polarity and strength. Both of these factors can influence the real shape of the molecule.
- Since VSEPR ignores molecular vibrations, it may not describe the behavior of the molecule with a great degree of accuracy. We usually employ orbital theory and computational chemistry in these cases to gain a more complete picture.
To conclude, the VSEPR theory is very useful for figuring out molecular geometry. It doesn t give a full picture in all cases, especially with transition metal complexes. But by considering the valence electrons, bonded atoms and charge on the central atom, we can quickly gain an idea of the expected geometry more often than not. The core principle behind the theory is that bonding electrons and lone pairs of electrons repel each other in a fairly predictable manner. Using our simple molecular geometry chart, you can figure out the shape of your molecule. In addition, you can get an idea of the bonding angles around the central atom.