Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.
Knowing Your Limits
Has anyone ever told you that you can do anything you want in life as long as you put your mind to it? Although it's true that hard work and determination will certainly take you far in life, unfortunately we all have some limits as to what we're able to accomplish. For example, if you're only five feet tall and weigh 120 pounds you probably won't end up succeeding as the next great offensive lineman in football, right? Now that doesn't mean you couldn't be the next leader of a country or a powerful businessman, it just means that your physical traits might limit you in being a rough-and-tumble football player.
Have you ever thought about the field of science, specifically chemistry, as having limits? As a human race we've done miraculous things in terms of technology, but we also still have limits that we are trying to get through in order to push the limits of research. In our lesson today, we're going to be talking about the limits associated with a bonding theory in chemistry called valence shell electron pair repulsion (VSEPR) theory. Let's get started!
What Is VSEPR?
VSEPR is a theory that chemists apply in order to determine the shape of a molecule. This principle says that in order to minimize repulsion interactions by electrons, molecules will adopt a shape in such a way that the bonds and lone pairs of electrons are as far away from each other as possible.
Chemical bonds are made up of electrons which are negatively charged. Since like-charges repel one another, the bonds and electrons want to have as much space in between them as possible so they aren't repelling one another. It's kind of like someone getting really close to you and 'getting in your bubble;' your natural reaction is to back up, right?
Let's use methane as an example. Methane contains four hydrogen atoms bonded to a central carbon atom. In order to maximize the distance between the four bonds in methane, it adopts a geometrical shape called a tetrahedron.
VSEPR theory can be used to predict the shape of virtually any molecule, provided we know how many bonds and lone pairs of electrons the central atoms contain. The term lone pairs is used to describe a pair or pairs of electrons that reside on the central atom but aren't involved in chemical bonding.
Limitations of VSEPR
Although VSEPR is a great tool for determining molecular shape, there are two big limitations to this theory.
1. Bond Angle Deviations
One of the main limitations of VSEPR theory is that it cannot predict the degree to which bond angles may deviate from idealized cases. For instance, let's look at methane and water side-by-side. VSEPR theory predicts that both molecules should have a tetrahedral shape since they both contain four groups of electrons around their central atoms (carbon for methane and oxygen for water).
Water actually adopts a bent shape instead of a tetrahedral shape. The fact that the bond angle for water is so different from methane is not something that VSEPR theory is able to predict or explain, especially from a numerical standpoint.
2. VSEPR Doesn't Support Isoelectronic Species
When two molecules have the exact same number of valence electrons (outer shell electrons) they're said to be isoelectronic and VSEPR predicts them to have the same shape. However, this isn't always the case. For example, let's look at iodine heptafluoride and the tellurium heptafluoride anion.
If we count up the total valence electrons, we get 56 electrons for each species. If we were to then consult a VSEPR table that gives relative shapes according to electron count we would find that each species would be predicted to be pentagonal bipyramidal. Experimental evidence, however, shows that indeed iodine heptafluoride conforms to the predicted shape but not in the case of tellurium heptafluoride.
In the case of tellurium heptafluoride, we notice a deviation in the Te-F bond angles and thus a different shape of this particular anion. VSEPR theory wouldn't predict the difference because both of the compounds have the same electron-count.
Okay, let's review what we've learned. VSEPR is a theory that chemists use to determine the shape of a molecule. This theory says that in order to minimize repulsion interactions by electrons, molecules will adopt a shape in such a way that the bonds and lone pairs of electrons are as far away from each other as possible.
VSEPR theory can be used to predict the shape of practically any molecule if we know how many bonds and lone pairs of electrons the central atoms contain. The term lone pairs describes a pair or pairs of electrons that reside on the central atom but aren't involved in chemical bonding. The two main limitations of VSEPR theory include:
- Bond Angle Deviations: In the cases of methane and water, VSEPR predicts them both to be tetrahedral in shape. However, because of the bent shape of water, the bond angle is far less than that of methane and is something that VSEPR theory can't account for numerically.
- Isoelectronic Species: Isoelectronic compounds means when two molecules have the exact same number of valence electrons, and in the case of iodine heptafluoride and the tellurium heptafluoride anion, VSEPR theory predicts them to both have a pentagonal bipyramidal shape. In reality, this is only true for iodine heptafluoride, with tellurium heptafluoride showing a deviation from this shape which would not be explained by VSEPR.
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